The Collaborative International Dictionary of English v.0.48:

Incumbent \In*cum"bent\, a. [L. incumbens, -entis, p. pr. of
   incumbere to lie down upon, press upon; pref. in- in, on +
   cumbere (in comp.); akin to cubare to lie down. See
   Incubate.]
   [1913 Webster]
   1. Lying; resting; reclining; recumbent; superimposed;
      superincumbent.
      [1913 Webster]

            Two incumbent figures, gracefully leaning upon it.
                                                  --Sir H.
                                                  Wotton.
      [1913 Webster]

            To move the incumbent load they try.  --Addison.
      [1913 Webster]

   2. Lying, resting, or imposed, as a duty or obligation;
      obligatory; always with on or upon.
      [1913 Webster]

            All men, truly zealous, will perform those good
            works that are incumbent on all Christians. --Sprat.
      [1913 Webster]

   3. (Bot.) Leaning or resting; -- said of anthers when lying
      on the inner side of the filament, or of cotyledons when
      the radicle lies against the back of one of them. --Gray.
      [1913 Webster]

   4. (Zool.) Bent downwards so that the ends touch, or rest on,
      something else; as, the incumbent toe of a bird.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Incumbent \In*cum"bent\, n.
   A person who is in present possession of a benefice or of any
   office.
   [1913 Webster]

         The incumbent lieth at the mercy of his patron.
                                                  --Swift.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incumbent
    adj 1: lying or leaning on something else; "an incumbent
           geological formation"
    2: necessary (for someone) as a duty or responsibility; morally
       binding; "it is incumbent on them to pay their own debts"
    3: currently holding an office; "the incumbent governor"
    n 1: the official who holds an office [syn: incumbent,
         officeholder]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "incumbent":
   addressee, artist-in-residence, beetle, beetle-browed, beetling,
   behooving, benefice-holder, beneficiary, binding, bridging,
   burdensome, chargeable to, commanding, compulsory, cumbersome,
   cumbrous, demanding, denizen, dweller, habitant, hirer, holdover,
   homesteader, house detective, imbricate, impendent, impending,
   incumbent on, inhabitant, inhabiter, inmate, inpatient, ins,
   intern, jack-in-office, jutting, lame duck, lapping, leaseholder,
   lessee, live-in maid, locum tenens, lodger, lowering, lumpish,
   mandatory, massive, necessary, new broom, obligatory, occupant,
   occupier, office-bearer, officeholder, official, onerous,
   oppressive, overarched, overhanging, overhung, overlapping,
   overlying, paying guest, pending, prescribed, president-elect,
   projecting, public official, public servant, renter, required,
   residencer, resident, resident physician, residentiary, resider,
   roomer, shingled, sojourner, spanning, squatter, sublessee,
   subtenant, superincumbent, tenant, tenant at sufferance,
   tenant for life, underlessee, unwieldy



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INCUMBENT, eccles. law. A clerk resident on his benefice with cure; he is so
called because he does, or ought to, bend the whole of his studies to his
duties. In common parlance, it signifies one who is in the possession of an
office, as, the present incumbent.



The Devil's Dictionary (1881-1906):

INCUMBENT, n.  A person of the liveliest interest to the outcumbents.