gaffer

From gaff (“hook”) + -er. * (cinema): The natural lighting on early film sets was adjusted by opening and closing flaps in the tent cloths, called gaff cloths or gaff flaps. * (glass)

Likely a contraction of godfather, but with the vowels influenced by grandfather. Compare French compère, German Gevatter. Compare also Old English ġefædera (“godfather”), of which some unattested dialectal descendant may have been an influence.