The Collaborative International Dictionary of English v.0.48:

Thrum \Thrum\, v. t.
   1. To play, as a stringed instrument, in a rude or monotonous
      manner.
      [1913 Webster]

   2. Hence, to drum on; to strike in a monotonous manner; to
      thrum the table.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thrum \Thrum\ (thr[u^]m), n. [OE. thrum, throm; akin to OD.
   drom, D. dreum, G. trumm, lump, end, fragment, OHG. drum end,
   Icel. [thorn]r["o]mr edge, brim, and L. terminus a limit,
   term. Cf. Term.] [Written also thrumb.]
   1. One of the ends of weaver's threads; hence, any soft,
      short threads or tufts resembling these.
      [1913 Webster]

   2. Any coarse yarn; an unraveled strand of rope.
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   3. (Bot.) A threadlike part of a flower; a stamen.
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   4. (Mining) A shove out of place; a small displacement or
      fault along a seam.
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   5. (Naut.) A mat made of canvas and tufts of yarn.
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   Thrum cap, a knitted cap. --Halliwell.

   Thrum hat, a hat made of coarse woolen cloth. --Minsheu.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thrum \Thrum\, v. t. [imp. & p. p. Thrummed; p. pr. & vb. n.
   Thrumming.]
   1. To furnish with thrums; to insert tufts in; to fringe.
      [1913 Webster]

            Are we born to thrum caps or pick straw? --Quarles.
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   2. (Naut.) To insert short pieces of rope-yarn or spun yarn
      in; as, to thrum a piece of canvas, or a mat, thus making
      a rough or tufted surface. --Totten.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thrum \Thrum\, v. i. [CF. Icel. ?ruma to rattle, to thunder, and
   E. drum.]
   1. To play rudely or monotonously on a stringed instrument
      with the fingers; to strum.
      [1913 Webster]

   2. Hence, to make a monotonous drumming noise; as, to thrum
      on a table.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thrum
    n 1: a thrumming sound; "he could hear the thrum of a banjo"
    v 1: sound with a monotonous hum [syn: hum, thrum]
    2: sound the strings of (a string instrument); "strum a guitar"
       [syn: strum, thrum]
    3: make a rhythmic sound; "Rain drummed against the windshield";
       "The drums beat all night" [syn: drum, beat, thrum]