costly

the word 'costly' is a native english formation, built upon the noun 'cost'. the root 'cost' entered middle english from old french 'coster', which itself derived from latin 'constare', literally 'to stand together, to stand at (a price)'. the latin verb is a compound of 'com-' (with) and 'stare' (to stand), from the proto-indo-european root sta- (to stand). the adjectival suffix '-ly', from old english '-lic', is a prolific suffix meaning 'like, having the nature of'. therefore, the literal construction 'cost-ly' means 'having the nature of (high) cost or expense'. over time, its meaning expanded beyond financial expense to encompass any serious detriment or sacrifice.

Structure: cost (price, expenditure) + ly (adjective-forming suffix) cost (from latin 'constare' meaning 'to stand at, to cost', via old french 'coster'; functions as the core noun/verb root) + ly (from old english '-lic', meaning 'having the qualities of'; functions as a suffix forming adjectives)