tap on (something)
1. To strike something lightly and swiftly. You're never going to drive the stake into the ground tapping on 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. The singer tapped on the microphone to see if it was one.
2. To strike someone or something lightly and swiftly on a particular spot or part. In this usage, a noun or pronoun is used between "tap" and "on." I turned around when someone tapped me on my shoulder. He always taps soda cans on their lids before opening them.
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.
tap someone or something on something
to make one or more light blows on some part of someone or something. Someone tapped me on the shoulder, and I turned around to see who it was. I tapped the drum on the top to find its pitch.
tap on something
to make one or more light blows on something. Who is that tapping on my windowpane? I wish you would stop tapping on the tabletop.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.