quash: [14] Quash goes back ultimately to Latin quatere ‘shake’ (source also of English rescue [14], which etymologically means ‘shake off, drive away’, and of concussion and percussion). From it evolved quassāre ‘shake to pieces, break’, which passed into Old French as quasser (its modern descendant is casser, from which English gets cashier ‘dismiss from the army’). English took quasser over as quash. Squash [16] comes ultimately from the Vulgar Latin derivative *exquassāre. => concussion, percussion, rescue, squash
quash (v.)
"to make void, annul," early 14c., from Old French quasser, casser "to annul, declare void," and directly from Medieval Latin quassare, alteration of Late Latin cassare, from cassus "null, void, empty" (see caste (n.)).
Meaning "to break, crush," is early 14c., from Old French quasser, casser "to break, smash, injure, harm, weaken," from Latin quassare "to shatter," frequentative of quatere (past participle quassus) "to shake," from PIE root *kwet- "to shake" (cognates: Greek passein "to sprinkle," Lithuanian kuteti "to shake up," Old Saxon skuddian "to move violently," German schütteln "to shake," Old English scudan "to hasten").
The words have influenced each other in form and sense since Medieval Latin and now are somewhat grown together. Related: Quashed; quashing.