The Collaborative International Dictionary of English v.0.48:

Half blood \Half" blood`\ (h[aum]f"bl[u^]d`).
   1. The relation between persons born of the same father or of
      the same mother, but not of both; as, a brother or sister
      of the half blood. See Blood, n., 2 and 4.
      [1913 Webster]

   2. A person so related to another.
      [1913 Webster]

   3. A person whose father and mother are of different races; a
      half-breed.
      [1913 Webster]

   Note: In the 2d and 3d senses usually with a hyphen.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blood \Blood\ (bl[u^]d), n. [OE. blod, blood, AS. bl[=o]d; akin
   to D. bloed, OHG. bluot, G. blut, Goth. bl[=o][thorn], Icel.
   bl[=o][eth], Sw. & Dan. blod; prob. fr. the same root as E.
   blow to bloom. See Blow to bloom.]
   1. The fluid which circulates in the principal vascular
      system of animals, carrying nourishment to all parts of
      the body, and bringing away waste products to be excreted.
      See under Arterial.
      [1913 Webster]

   Note: The blood consists of a liquid, the plasma, containing
         minute particles, the blood corpuscles. In the
         invertebrate animals it is usually nearly colorless,
         and contains only one kind of corpuscles; but in all
         vertebrates, except Amphioxus, it contains some
         colorless corpuscles, with many more which are red and
         give the blood its uniformly red color. See
         Corpuscle, Plasma.
         [1913 Webster]

   2. Relationship by descent from a common ancestor;
      consanguinity; kinship.
      [1913 Webster]

            To share the blood of Saxon royalty.  --Sir W.
                                                  Scott.
      [1913 Webster]

            A friend of our own blood.            --Waller.
      [1913 Webster]

   Half blood (Law), relationship through only one parent.

   Whole blood, relationship through both father and mother.
      In American Law, blood includes both half blood, and whole
      blood. --Bouvier. --Peters.
      [1913 Webster]

   3. Descent; lineage; especially, honorable birth; the highest
      royal lineage.
      [1913 Webster]

            Give us a prince of blood, a son of Priam. --Shak.
      [1913 Webster]

            I am a gentleman of blood and breeding. --Shak.
      [1913 Webster]

   4. (Stock Breeding) Descent from parents of recognized breed;
      excellence or purity of breed.
      [1913 Webster]

   Note: In stock breeding half blood is descent showing one
         half only of pure breed. Blue blood, full blood, or
         warm blood, is the same as blood.
         [1913 Webster]

   5. The fleshy nature of man.
      [1913 Webster]

            Nor gives it satisfaction to our blood. --Shak.
      [1913 Webster]

   6. The shedding of blood; the taking of life, murder;
      manslaughter; destruction.
      [1913 Webster]

            So wills the fierce, avenging sprite,
            Till blood for blood atones.          --Hood.
      [1913 Webster]

   7. A bloodthirsty or murderous disposition. [R.]
      [1913 Webster]

            He was a thing of blood, whose every motion
            Was timed with dying cries.           --Shak.
      [1913 Webster]

   8. Temper of mind; disposition; state of the passions; -- as
      if the blood were the seat of emotions.
      [1913 Webster]

            When you perceive his blood inclined to mirth.
                                                  --Shak.
      [1913 Webster]

   Note: Often, in this sense, accompanied with bad, cold, warm,
         or other qualifying word. Thus, to commit an act in
         cold blood, is to do it deliberately, and without
         sudden passion; to do it in bad blood, is to do it in
         anger. Warm blood denotes a temper inflamed or
         irritated. To warm or heat the blood is to excite the
         passions. Qualified by up, excited feeling or passion
         is signified; as, my blood was up.
         [1913 Webster]

   9. A man of fire or spirit; a fiery spark; a gay, showy man;
      a rake.
      [1913 Webster]

            Seest thou not . . . how giddily 'a turns about all
            the hot bloods between fourteen and five and thirty?
                                                  --Shak.
      [1913 Webster]

            It was the morning costume of a dandy or blood.
                                                  --Thackeray.
      [1913 Webster]

   10. The juice of anything, especially if red.
       [1913 Webster]

             He washed . . . his clothes in the blood of grapes.
                                                  --Gen. xiix.
                                                  11.
       [1913 Webster]

   Note: Blood is often used as an adjective, and as the first
         part of self-explaining compound words; as,
         blood-bespotted, blood-bought, blood-curdling,
         blood-dyed, blood-red, blood-spilling, blood-stained,
         blood-warm, blood-won.
         [1913 Webster]

   Blood baptism (Eccl. Hist.), the martyrdom of those who had
      not been baptized. They were considered as baptized in
      blood, and this was regarded as a full substitute for
      literal baptism.

   Blood blister, a blister or bleb containing blood or bloody
      serum, usually caused by an injury.

   Blood brother, brother by blood or birth.

   Blood clam (Zool.), a bivalve mollusk of the genus Arca and
      allied genera, esp. Argina pexata of the American coast.
      So named from the color of its flesh.

   Blood corpuscle. See Corpuscle.

   Blood crystal (Physiol.), one of the crystals formed by the
      separation in a crystalline form of the h[ae]moglobin of
      the red blood corpuscles; h[ae]matocrystallin. All blood
      does not yield blood crystals.

   Blood heat, heat equal to the temperature of human blood,
      or about 981/2 [deg] Fahr.

   Blood horse, a horse whose blood or lineage is derived from
      the purest and most highly prized origin or stock.

   Blood money. See in the Vocabulary.

   Blood orange, an orange with dark red pulp.

   Blood poisoning (Med.), a morbid state of the blood caused
      by the introduction of poisonous or infective matters from
      without, or the absorption or retention of such as are
      produced in the body itself; tox[ae]mia.

   Blood pudding, a pudding made of blood and other materials.


   Blood relation, one connected by blood or descent.

   Blood spavin. See under Spavin.

   Blood vessel. See in the Vocabulary.

   Blue blood, the blood of noble or aristocratic families,
      which, according to a Spanish prover, has in it a tinge of
      blue; -- hence, a member of an old and aristocratic
      family.

   Flesh and blood.
       (a) A blood relation, esp. a child.
       (b) Human nature.

   In blood (Hunting), in a state of perfect health and vigor.
      --Shak.

   To let blood. See under Let.

   Prince of the blood, the son of a sovereign, or the issue
      of a royal family. The sons, brothers, and uncles of the
      sovereign are styled princes of the blood royal; and the
      daughters, sisters, and aunts are princesses of the blood
      royal.
      [1913 Webster]

WordNet (r) 3.0 (2006):

half blood
    n 1: one of siblings who have only one parent in common