No one knows for sure where this came from, but its similarity to the now obsolete mouldwarp ‘mole’ [14] (a compound noun whose etymological meaning is ‘earththrower’) suggests that it could represent a truncated version of mouldwarp’s prehistoric Germanic ancestor. The metaphorical application of the word to a ‘traitor working secretly’ has been traced back as far as the 17th century, but its modern currency is due to its use by the British espionage writer John le Carré. Mole ‘harbour wall’ [16] comes via French môle and medieval Greek mólos from Latin mōlēs ‘mass, massive structure’.
The diminutive form of this, coined in modern times, is mōlēcula, from which, via French molécule, English gets molecule [18]. Other relatives are demolish and, possibly, molest [14], which comes ultimately from Latin molestus ‘troublesome’, connected by some scholars with mōlēs. And German mol, a convenient shortening of molekulargewicht ‘molecular weight’, has given English its fourth mole [20], used as the basic unit of measurement for the amount of a substance.