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."