heel-tap (n.)

also heeltap, 1680s, "one of the bits of leather that are stacked up to make a shoe heel" (see heel (n.1)); meaning "bit of liquor left in a glass or bottle" first recorded 1767; the exact connection is uncertain unless it be "the last or final part." Related: Heeltaps.

A jolly dog, is one who has no conversation in company, but "fill about, what's the toast, damn your heel-taps," and roars out an obscene ballad when he gets drunk. [The Batchelor, March 28, 1767]