Real Dictionary

-proof
combining form: -proof combines with nouns and verbs to form adjectives which indicate that something cannot be damaged or badly affected by the thing or action mentioned.

-proof in American English
impervious to
combining form: a combining form meaning “resistant, impervious to” that which is specified by the initial element

-proof in British English
combining form: secure against (damage by); (make) impervious to

proof
Proof is a fact, argument, or piece of evidence which shows that something is definitely true or definitely exists.

proof in American English
the act or process of proving; a testing or trying of something
adjective: able to withstand; successful in not being overcome
noun: evidence sufficient to establish a thing as true, or to produce belief in its truth
transitive verb: to test; examine for flaws, errors, etc.; check against a standard or standards

proof in British English
adjective: able to resist; impervious (to)
noun: any evidence that establishes or helps to establish the truth, validity, quality, etc, of something
verb: to take a proof from (type matter, a plate, etc)