Source : Webster's Revised Unabridged Dictionary (1913)

Strum \Strum\, v. t. & i. [imp. & p. p. {Strummed}; p. pr. & vb.
   n. {Strumming}.] [Probably of imitative origin. Cf. {Thrum}.]
   To play on an instrument of music, or as on an instrument, in
   an unskillful or noisy way; to thrum; as, to strum a piano.

Source : WordNet®

strum
     n : sound of strumming; "the strum of a guitar"
     [also: {strumming}, {strummed}]

strum
     v : sound the strings of (a string instrument); "strum a guitar"
         [syn: {thrum}]
     [also: {strumming}, {strummed}]