/moʊl/
[noun] A small, burrowing mammal with dark velvety fur, tiny eyes, and strong front claws that lives underground; also used to describe a spy who works secretly inside an organisation, or a small dark spot on the skin.
The word 'mole' for a spy was popularised by John le Carré, but the animal it borrows from has been digging through English for over 600 years.
The mole as a small, tunnelling mammal enters English in the 14th century, most likely from Middle Dutch 'mol' or Middle Low German 'mol' or 'mul', which referred to the same creature. These Germanic forms are thought to descend from an older Proto-Germanic root, possibly related to a word meaning 'to grind' or 'to dig into the earth', reflecting the animal's most defining habit. Old English itself used a different word — 'wand-weorp', literally meaning 'earth-thrower' — but this was eventually replaced entirely by the Germanic borrowing, showing how the language absorbed words from trading neighbours across the North Sea.
The mole as a dark spot or blemish on the skin has a completely separate origin. This word comes from Old English 'māl', meaning a spot, mark, or stain, which is related to Old High German 'meil' and Gothic 'mail', both carrying similar meanings. This 'māl' was part of a family of Germanic words connected to marking or discolouring, and it passed smoothly from Old English into Middle English with barely a change in form or meaning. The two 'moles' — the animal and the skin mark — are therefore etymological strangers that simply happen to sound alike in modern English.
The sense of 'mole' meaning a spy, or a person who secretly passes information to an enemy, is surprisingly recent. It was brought into popular use by the British novelist John le Carré in his 1974 spy thriller 'Tinker Tailor Soldier Spy', though le Carré himself credited the term to earlier intelligence jargon. The metaphor is vivid and perfectly chosen: like the animal, a mole in an organisation works in darkness, moves slowly beneath the surface, and causes damage that is only visible long after the work has been done.
There is also a scientific 'mole', the unit used in chemistry to measure an amount of substance, equal to approximately 6.022 × 10²³ particles. This word is borrowed directly from German 'Mol', which was coined in the late 19th century as a short form of 'Molekulargewicht', meaning 'molecular weight'. The German 'Molekül' came from Modern Latin 'molecula', a diminutive of Latin 'moles', meaning 'mass' or 'pile'. So the chain runs: Latin 'moles' → Modern Latin 'molecula' → German 'Molekül' → German 'Mol' → English 'mole'. This scientific meaning shares no ancestry with either the animal or the skin spot.
Finally, there is the culinary 'mole', the rich, dark Mexican sauce made with chillies, spices, and often chocolate. This word comes from Nahuatl 'molli' or 'mulli', meaning 'sauce' or 'chilli sauce', the language of the Aztec people. It entered English through Mexican Spanish 'mole', and represents one of the many words English borrowed from the indigenous languages of the Americas via Spanish. This fourth 'mole' reminds us that English is not just a European language — it has absorbed words from cultures across the entire world, and a single short syllable can contain multitudes of history.