c.1200, from Old English þeah, and in part from Old Norse þo "though," both from Proto-Germanic *thaukh (cf. Gothic þauh, Old Frisian thach, Middle Dutch, Dutch doch, Old High German doh, German doch), from PIE demonstrative pronoun *to- (see that). The evolution of the terminal sound did not follow laugh, tough, etc., though a tendency to end the word in "f" existed c.1300-1750 and persists in dialects.
