tap at

tap at (something)

To strike something lightly, swiftly, and repeatedly. You're never going to drive the stake into the ground tapping at it with your hammer like that. I could have sworn I heard someone tapping at the window, but there was no one there when I got up to check.
See also: tap
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

tap at something

to make one or more light blows on something. Fred tapped at the door, but no one heard him. Who is tapping at my window?
See also: tap
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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References in classic literature ?
Bucketful by bucketful, from the tap at the sink in the corner, he filled a large galvanized-iron tub.
Wineries fill kegs with premium wine and restaurants dispense the high-quality product via a specially made tap at the bar.
Two, eliminating overshoot, or "coast," of the tap at the bottom of the thread during the spindle breaking portion of the tapping cycle.
Opportunities for live performances declined for tappers in the 1950s, and Edwards taught tap at the Philadelphia School of Dance Arts.
This can cause a slight "pull-out" of the tap at the end of each cycle, resulting in loss of depth control.
Carbide offers the potential of higher cutting speeds and lifetimes of 100 to 500 times the life of a conventional tap at about ten times its cost, Matysiak says.