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.
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.
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").
4
unit of molecular quantity, 1902, from German Mol coined 1900 by German chemist Wilhelm Ostwald (1853-1912), short for Molekül (see molecule).