"betroth (two people), bind in wedlock; pledge oneself to," early 12c., from Old English handfæsten and cognate Old Norse handfesta "to pledge, betroth; strike a bargain by shaking hands;" for first element see hand (n.); second element is from Proto-Germanic causative verb *fastjan "to make firm," from PIE *past- "solid, firm" (see fast (adj.). Related: Handfasted; handfasting. The noun in Old English was >handfæstung.
