As plankton said v
there was an older meaning of "prove" which was "test". Certainly the German for "test" is "prufen", which was in use in an Aglicised way. The phrase was coined with this in mind, then the meaning of "prove" changed to something more definite. Dunno how that happened though.
( ,
Thu 5 Apr 2007, 15:02,
archived)