means To tap is to strike something lightly, or it's the valve-controlled spout that lets a liquid out of a pipe, cask, or barrel on demand.

from Two threads twist together here. The 'gentle strike' sense comes from Old English tæppian, an echoic word — it sounds like the small knock it names, a cousin of similar tapping words across the Germanic family. The 'spout' sense descends from Old English tæppa, the peg or stopper plugging a cask; to 'tap' a barrel meant to fit it with that pin so the ale could be drawn. From the tavern peg we got the 'tap room,' and from the act of drawing liquid we eventually got the modern faucet that gives the essence its quiet heroism.