tap (v.1)

"strike lightly," c. 1200, from Old French taper "tap, rap, strike" (12c.), from a Gallo-Roman or Germanic source ultimately imitative of the sound of rapping. Meaning "to designate for some duty or for membership" is recorded from 1952, from notion of a tap on the shoulder. Related: Tapped; tapping.

tap (n.1)

"stopper, faucet through which liquid can be drawn," Old English tæppa "tap, spigot," from Proto-Germanic *tappon (source also of Middle Dutch tappe, Dutch tap, Old High German zapfo, German Zapfe). Originally a tapering cylindrical peg for a cask, then a hollowed one to draw from it (compare sense evolution of spigot). Phrase on tap "ready for use, ready to be drawn and served" is recorded from late 15c. Tap-wrench, used in turning one, attested from 1815.

tap (v.2)

"to supply with a tap," late Old English tæppian, from source of tap (n.1); compare German zapfen "to tap." Meaning "to draw liquor with a tap" is from mid-15c. Extended sense "make use of" is first recorded 1570s. Meaning "listen in secretly" (1869), originally was with reference to telegraph wires. Tapped out "broke" is 1940s slang, perhaps from the notion of having tapped all one's acquaintances for loans already (compare British slang on the tap "begging, making requests for loans," 1932).

tap (n.2)

"light blow or stroke," mid-14c., from tap (v.1). Tap dancer first recorded 1927, from tap (n.) in the sense of "metal plate over the heel of a shoe" (1680s).

tap (n.3)

"device to listen in secretly on telephone calls," 1923, from tap (v.2) in the "listen secretly" sense.

Definitions of tap
tap (v.)
cut a female screw thread with a tap;
tap (v.)
draw from or dip into to get something;
tap one's memory
tap a source of money
tap (v.)
strike lightly;
He tapped me on the shoulder
Synonyms: tip
tap (v.)
draw from; make good use of;
Synonyms: exploit
tap (v.)
tap a telephone or telegraph wire to get information;
The FBI was tapping the phone line of the suspected spy
Synonyms: wiretap / intercept / bug
tap (v.)
furnish with a tap or spout, so as to be able to draw liquid from it;
tap a cask of wine
tap (v.)
make light, repeated taps on a surface;
he was tapping his fingers on the table impatiently
Synonyms: rap / knock / pink
tap (v.)
walk with a tapping sound;
tap (v.)
dance and make rhythmic clicking sounds by means of metal plates nailed to the sole of the dance shoes;
Synonyms: tapdance
tap (v.)
draw (liquor) from a tap;
tap beer in a bar
tap (v.)
pierce in order to draw a liquid from;
tap a maple tree for its syrup
tap a keg of beer
tap (v.)
make a solicitation or entreaty for something; request urgently or persistently;
Synonyms: solicit / beg
tap (n.)
the sound made by a gentle blow;
Synonyms: pat / rap
tap (n.)
a gentle blow;
Synonyms: rap / strike
tap (n.)
a faucet for drawing water from a pipe or cask;
Synonyms: water faucet / water tap / hydrant
tap (n.)
a small metal plate that attaches to the toe or heel of a shoe (as in tap dancing);
tap (n.)
a tool for cutting female (internal) screw threads;
tap (n.)
a plug for a bunghole in a cask;
Synonyms: spigot
tap (n.)
the act of tapping a telephone or telegraph line to get information;
Synonyms: wiretap
tap (n.)
a light touch or stroke;
Synonyms: pat / dab