waterproof adj.

Old English wæter, from Proto-Germanic *watr- (source also of Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, Old Norse vatn, Gothic wato "water"), from PIE *wod-or, suffixed form of root *wed- (1) "water; wet."

To keep (one's) head above water in the figurative sense is recorded from 1742. Water cooler is recorded from 1846; water polo from 1884; water torture from 1928. Linguists believe PIE had two root words for water: *ap- and *wed-. The first (preserved in Sanskrit apah as well as Punjab and julep) was "animate," referring to water as a living force; the latter referred to it as an inanimate substance. The same probably was true of fire (n.).

c. 1200, preove "evidence and argumentation to establish the fact of (something) beyond reasonable doubt," from Anglo-French prove, preove, Old French proeve, prueve "proof, test, experience" (13c., Modern French preuve), from Late Latin proba "a proof," a back-formation from Latin probare "to prove" (see prove). "The devocalization of v to f ensued upon the loss of final e; cf. the relation of v and f in believe, belief, relieve, relief, behove, behoof, etc." [OED].

The meaning "act of proving" is early 14c. The meaning "act of testing or making trial of anything" is from late 14c., from influence of prove. Meaning "standard of strength of distilled liquor" is from 1705, on the notion of "having been tested as to degree of strength." The use in photography is from 1855. The typographical sense of "trial impression to test type" is from c. 1600. The numismatic sense of "coin struck to test a die" is from 1762; now mostly in reference to coins struck from highly polished dies, mainly for collectors.

The adjectival sense "impenetrable, able to resist" (as in proof against) is recorded from 1590s, from the noun in expressions such as proof of (mid-15c.), hence extended senses involving "of tested power against" in compounds such as fireproof (1630s), rust-proof (1690s), waterproof (1725), fool-proof (1902), etc. Shakespeare has shame-proof. Expression the proof is in the pudding (1915) is a curious perversion of earlier proof of the pudding shall be in the eating (1708), with proof in the sense "quality of proving good or turning out well" (17c.).