proof [ME] This came via Old French proeve from Latin probare, ‘to test or prove’. Proof spirit or 100 per cent proof spirit was originally defined as a solution of alcohol that will ignite when mixed with gunpowder—in Britain this meant an alcohol content of 57.07 per cent. In the expression the proof of the pudding is in the eating, proof is used in the sense ‘test’ rather than ‘verification, proving to be true’. Probare is also the source of prove [ME], probe [LME], probate [LME] where you have to prove the will in law, and probation [LME] which is a form of testing.