The Collaborative International Dictionary of English v.0.48:

Trepan \Tre*pan"\, v. t. & i. [imp. & p. p. Trepanned; p. pr.
   & vb. n. Trepanning.] [Cf. F. tr['e]paner. See Trepan a
   saw.] (Surg.)
   To perforate (the skull) with a trepan, so as to remove a
   portion of the bone, and thus relieve the brain from pressure
   or irritation; to perform an operation with the trepan.
   [1913 Webster]