1610s, "of or pertaining to or meant for a pocket," from pocket (n.). Pocket-money "money for occasional or trivial purposes" is attested from 1630s; pocket-handkerchief is from 1640s. Often merely implying a small-sized version of something (for example of of warships, from 1930; also compare Pocket Venus "beautiful, small woman," attested from 1808). Pocket veto attested from 1842, American English.
The "pocket veto" can operate only in the case of bills sent to the President within ten days of Congressional adjournment. If he retain such a bill (figuratively, in his pocket) neither giving it his sanction by signing it, nor withholding his sanction in returning it to Congress, the bill is defeated. The President is not bound to give reasons for defeating a bill by a pocket veto which he has not had at least ten days to consider. In a regular veto he is bound to give such reasons. [James Albert Woodburn, "The American Republic and its Government," Putnam's, 1903]
In English history a pocket borough (by 1798) was one whose parliamentary representation was under the control of one person or family.
BRAMBER, Sussex. This is one of the burgage-tenure or nomination boroughs. The place altogether consists only of twenty-two miserable thatched cottages, and is composed of two intersections of a street, the upper and middle parts of which constitute another pocket borough, called Steyning, which we shall notice in the second class, as belonging to the Duke of Norfolk. ["A Key to the House of Commons," London, 1820]
the modern English word is a merger of two words, both in Middle English as quashen, from two unrelated Latin verbs.
1. "to suppress, overcome" (mid-13c.); "to make void, annul, nullify, veto" (mid-14c.), from Old French quasser, quassier, casser "to annul, declare void," and directly from Medieval Latin quassare, alteration of Late Latin cassare, from cassus "null, void, empty" (from extended form of PIE root *kes- "to cut"). The meaning "subdue, put down summarily" is from c. 1600.
2. "to break, crush, beat to pieces" early 14c., from Old French quasser, casser "to break, smash, destroy; maltreat, injure, harm, weaken," from Latin quassare "to shatter, shake or toss violently," frequentative of quatere (past participle quassus) "to shake," from PIE root *kwet- "to shake" (source also of Greek passein "to sprinkle," Lithuanian kutėti "to shake up," Old Saxon skuddian "to move violently," German schütteln "to shake," Old English scudan "to hasten").
In Medieval Latin, quassare often was used for cassare, and in later French the form of both words is casser. The words in English now are somewhat, or entirely, fused. Related: Quashed; quashing.