1.) T' practice o' modifyin' a rocket kit t' achieve some result other than that intended by t' manufacturer;
2.) Takin' two (or more) kits and combinin' ("bashing") them into a new design. Often used as a contest event (Team Kitbash, where teams compete, shiver me timbers, Kitbash Duration, shiver me timbers, Scale Kitbash, etc) where t' idea is t' be creative in a limited amount o' time. Avast! See Kitbash Articles.