pun: [17] Snappy monosyllables produced by breaking off a piece of a longer word were all the rage in late 17th- and early 18th-century England (mob is a well-known example), and it is thought that pun may be one of them. It seems to be short for pundigrion, a short-lived fanciful 17th- and 18th-century term for a ‘pun’ or ‘quibble’ which may have been adapted from Italian puntiglio ‘nice point, quibble’ (source of English punctilious).
pun (n.)
1660s (first attested in Dryden), of uncertain origin, perhaps from pundigron, which is perhaps a humorous alteration of Italian puntiglio "equivocation, trivial objection," diminutive of Latin punctum "point." This is pure speculation. The verb also is attested from 1660s. Related: Punned; punning.
Pun was prob. one of the clipped words, such as cit, mob, nob, snob, which came into fashionable slang at or after the Restoration. [OED]