A letterhack is a copious contributor to lettercols. An important term in early fandom and during WWII, when muchfannish interaction was in prozine letter columns, but later it referred mainly to fen who locced fanzines. Letterhack implies a much more intensive, crifannish approach to loccing than mere loccer.

One might be a letterhack to one particular fanzine (Title letterhacks tended to be so singular) or to many zines. The most famed letterhacks in fanzine fandom were Harry Warner, Jr. and Mike Glicksohn, of whom it was said that a faned hadn't published a real fanzine unless it had locs from both.

Though mainly a noun, letterhack is occasionally verbified (“Joe Fan letterhacked his way to BNFhood”).

from Fancyclopedia 2 ca. 1959
A fan who seeks egoboo by writing numerous letters of comment and criticism to the prozines. A harmless druj.