The Collaborative International Dictionary of English v.0.48:

Nettle \Net"tle\, n. [AS. netele; akin to D. netel, G. nessel,
   OHG. nezz["i]la, nazza, Dan. nelde, n[aum]lde, Sw.
   n[aum]ssla; cf, Lith. notere.] (Bot.)
   A plant of the genus Urtica, covered with minute sharp
   hairs containing a poison that produces a stinging sensation.
   Urtica gracilis is common in the Northern, and Urtica
   chamaedryoides in the Southern, United States. The common
   European species, Urtica urens and Urtica dioica, are
   also found in the Eastern united States. Urtica pilulifera
   is the Roman nettle of England.
   [1913 Webster]

   Note: The term nettle has been given to many plants related
         to, or to some way resembling, the true nettle; as:

   Australian nettle, a stinging tree or shrub of the genus
      Laportea (as Laportea gigas and Laportea moroides);
      -- also called nettle tree.

   Bee nettle, Hemp nettle, a species of Galeopsis. See
      under Hemp.

   Blind nettle, Dead nettle, a harmless species of
      Lamium.

   False nettle (Baehmeria cylindrica), a plant common in
      the United States, and related to the true nettles.

   Hedge nettle, a species of Stachys. See under Hedge.

   Horse nettle (Solanum Carolinense). See under Horse.

   nettle tree.
   (a) Same as Hackberry.
   (b) See Australian nettle (above).

   Spurge nettle, a stinging American herb of the Spurge
      family (Jatropha urens).

   Wood nettle, a plant (Laportea Canadensis) which stings
      severely, and is related to the true nettles.
      [1913 Webster]

   Nettle cloth, a kind of thick cotton stuff, japanned, and
      used as a substitute for leather for various purposes.

   Nettle rash (Med.), an eruptive disease resembling the
      effects of whipping with nettles.

   Sea nettle (Zool.), a medusa.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hemp \Hemp\ (h[e^]mp), n. [OE. hemp, AS. henep, h[ae]nep; akin
   to D. hennep, OHG. hanaf, G. hanf, Icel. hampr, Dan. hamp,
   Sw. hampa, L. cannabis, cannabum, Gr. ka`nnabis, ka`nnabos;
   cf. Russ. konoplia, Skr. [,c]a[.n]a; all prob. borrowed from
   some other language at an early time. Cf. Cannabine,
   Canvas.]
   1. (Bot.) A plant of the genus Cannabis (Cannabis
      sativa), the fibrous skin or bark of which is used for
      making cloth and cordage. The name is also applied to
      various other plants yielding fiber.
      [1913 Webster]

   2. The fiber of the skin or rind of the plant, prepared for
      spinning. The name has also been extended to various
      fibers resembling the true hemp.
      [1913 Webster]

   African hemp, Bowstring hemp. See under African, and
      Bowstring.

   Bastard hemp, the Asiatic herb Datisca cannabina.

   Canada hemp, a species of dogbane (Apocynum cannabinum),
      the fiber of which was used by the Indians.

   Hemp agrimony, a coarse, composite herb of Europe
      (Eupatorium cannabinum), much like the American boneset.


   Hemp nettle, a plant of the genus Galeopsis (Galeopsis
      Tetrahit), belonging to the Mint family.

   Indian hemp. See under Indian, a.

   Manila hemp, the fiber of Musa textilis.

   Sisal hemp, the fiber of Agave sisalana, of Mexico and
      Yucatan.

   Sunn hemp, a fiber obtained from a leguminous plant
      (Crotalaria juncea).

   Water hemp, an annual American weed (Acnida cannabina),
      related to the amaranth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hemp nettle
    n 1: coarse bristly Eurasian plant with white or reddish flowers
         and foliage resembling that of a nettle; common as a weed
         in United States [syn: hemp nettle, dead nettle,
         Galeopsis tetrahit]