mole (n.1)
spot on skin, Old English mal "spot, mark, blemish," especially on cloth or linen, from Proto-Germanic *mailan "spot, mark" (cognates: Old High German meil, German Mal, Gothic mail "wrinkle"), from PIE root *mai- "to stain, defile" (cognates: Greek miainein "to stain, defile," see miasma). Specifically of dark marks on human skin from late 14c.
mole (n.2)
type of small burrowing mammal (Talpa europea), mid-14c., probably from obsolete moldwarp, literally "earth-thrower." Spy sense first recorded 1974 in John le Carré (but suggested from early 20c.), from notion of "burrowing." Metaphoric use for "one who works in darkness" is from c.1600.
mole (n.3)
"breakwater," 1540s, from Middle French môle "breakwater" (16c.), ultimately from Latin moles "mass, massive structure, barrier," from PIE root *mo- "to exert oneself" (cognates: Greek molos "effort," molis "hardly, scarcely;" German mühen "to tire," müde "weary, tired;" Russian majat' "to fatigue, exhaust," maja "hard work").
mole (n.4)
unit of molecular quantity, 1902, from German Mol coined 1900 by German chemist Wilhelm Ostwald (1853-1912), short for Molekül (see molecule).