Old English hider, from Proto-Germanic *hithra- (source also of Old Norse heðra "here," Gothic hidre "hither"), from PIE *kitro-, suffixed variant form of root *ko-, the stem of demonstrative pronoun meaning "this" (compare here). Spelling change from -d- to -th- is the same evolution seen in father, etc. Relation to here is the same as that of thither to there.
