wire-tapping (n.)

c. 1400, tappen, "draw (liquor) from a tap; draw and sell (ale) in small quantities," from tap (n.1)); also compare late Old English tæppian "to supply with a tap," and German zapfen "to tap."

The extended sense "open up in order to make use of" is attested by 1570s. The meaning "listen in secretly" (1869), originally was in reference to telegraph wires, on the notion of "piercing so as to draw something out."

Related: Tapped; tapping. Tapped out "broke" is 1940s slang, perhaps from the notion of already having "tapped" all one's acquaintances for loans (compare British slang on the tap "begging, making requests for loans," 1932). The notion of "tapping" (on the shoulder, to get attention) also might be involved, from tap (v.1).

The meaning "apply a thickness of leather to" (a sole or heel of a boot or shoe), is by 1746.

Old English wir "metal drawn out into a fine thread," from Proto-Germanic *wira- (source also of Old Norse viravirka "filigree work," Swedish vira "to twist," Old High German wiara "fine gold work"), from PIE root *wei- "to turn, twist, plait."

A wire as marking the finish line of a racecourse is attested from 1883; hence the figurative down to the wire. Wire-puller in the political sense is by 1842, American English, on the image of pulling the wires that work a puppet; the image itself in politics is older:

The ministerial majority being thus reduced to five in a house of five hundred and eighty-three, Lord John Russell and Lord Melbourne respectively announce the breaking up of the administration, and the curtain falls on the first act of the political farce, to the infinite annoyance and surprise of the prime wire-puller in the puppet-show. [British and Foreign Review, vol. IX, July-October 1839]

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