Is it farther or further?
c.1400, from French trépaner (14c.), from Medieval Latin trepanum "a saw for cutting out small pieces of bone from the skull," from Greek trypanon, from trypan "to bore," related to trype "hole" (cf. Old Church Slavonic truplu "hollow"), from PIE root *tere- "to rub, turn" (see throw). Related: Trepanned; trepanning.
trepan tre·pan (trĭ-pān')
A trephine. v. tre·panned, tre·pan·ning, tre·pans