Light, quick hits or touches on something, or the plural of tap (a device that controls water flow).
From Old Norse 'tappa' meaning 'to strike lightly.' The word split into multiple meanings: the hitting sense comes from the sound itself (onomatopoeia), while the water valve sense developed later as a name for the device.
The bugle call 'Taps' played at military funerals was originally a drum signal meaning 'put out the lights'—it comes from the tap-tap-tap rhythm of the snare drum.
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