mole (n.1)

spot on skin, Old English mal "spot, mark, blemish," especially on cloth or linen, from Proto-Germanic *mailan "spot, mark" (source also of Old High German meil, German Mal, Gothic mail "wrinkle"), from PIE root *mai- (2) "to stain, soil, defile" (source also of Greek miainein "to stain, defile," see miasma). Specifically of small, permanent dark marks on human skin from late 14c.

type of small burrowing insectivorous mammal (genus Talpa), mid-14c., molle (early 13c. in surnames); perhaps a shortening of obsolete moldwarp, literally "earth-thrower," but this sort of abbreviation is rare at that early age, and perhaps it is rather directly from the root of mold (n.3) "loose earth." It may represent an unrecorded Old English word; compare Middle Dutch mol, molle, Middle Low German mol, mul.

From c. 1600 as a figure of "one who works in darkness" (in Middle English, moldewerpe was figurative of a cleric overly concerned with worldly things). The espionage sense of "secret agent who gradually attains a position deep within organization or nation was popularized 1974 in John le Carré (but suggested from early 20c.), from the notion of "burrowing."

"massive structure used as a breakwater," 1540s, from Middle French môle "breakwater" (16c.), ultimately from Latin moles "mass, massive structure, barrier," perhaps from PIE root *mō- "to exert oneself" (source also of Greek molos "effort," molis "hardly, scarcely;" German mühen "to tire," müde "weary, tired;" Russian majat' "to fatigue, exhaust," maja "hard work").

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