A term used by improvisers (those who perfrom improv/improvisation), to describe the act of improvising material at any time prior to the actual performance (bad), or planning how the performance of improvised material plays out (worse). In either case, pre-prov-ing (pre-provving?) renders the pre-prov-ed (pre-provved?) material un-improvisable, and thus null, void, and unapproachable to any good sport of a good improviser. Improv practice is intentional pre-prov, in order to practice skills and techniques. When just joking around, yes, unintentional pre-prov happens, and though often funny, may present a major bummer to any good sport of a good improviser. Though often adaptable to create quality, finalized, REHEARSED material (i.e., a practiced comedy sketch, a polished song), what is pre-prov-ed/pre-provved can never be performed as true improv (without involving memory loss, that is).
Note: What one performer pre-provs may still be original to another, and still qualify as improv, providing that the second performer did not know of the first's pre-prov bummer.