strum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To play (a stringed musical instrument) by stroking or brushing the strings.
• intransitive verb To play (music) on a stringed instrument in this way.
• intransitive verb To play a stringed instrument by strumming.
• noun The act or sound of strumming.

from The Century Dictionary.

• To play unskilfully, or in a vulgar, noisy manner, on a stringed musical instrument of the lute or harp kind, as a guitar, banjo, or zither, or (by extension) on a pianoforte; thrum.
• To play upon carelessly or unskilfully, as a stringed instrument; produce by rough manipulation of musical chords.
• To produce a specified effect upon by strumming on a musical instrument.
• noun A strumming; a careless or discordant performance on a stringed instrument.

from the GNU version of the Collaborative International Dictionary of English.

• verb To play on an instrument of music, or as on an instrument, in an unskillful or noisy way; to thrum.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb To play a guitar or other stringed instrument using various strings simultaneously.
• noun The sound made by playing various strings of a stringed instrument simultaneously.
• noun The act of strumming.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb sound the strings of (a string instrument)
• noun sound of strumming