Playing a stringed instrument (like a guitar) by brushing your fingers across the strings rhythmically rather than plucking individual notes.
From Middle English 'strum,' possibly imitative of the sound, or related to 'string'; first recorded in the 1600s as a somewhat informal way to play instruments.
Strumming sounds like what it is—the word itself mimics the fuzzy, rhythmic brushing sound, which is why people instinctively know what you mean even if they've never heard the term before!
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