Abarim Publications' online Biblical Hebrew Dictionary
רבק
The verb רבק (rbq), meaning to tie fast, isn't used in the Bible, but cognate languages have it, so scholars assume it once existed; it just managed to escape the compass of the Biblical narrative.
Another sure clue that this verb once existed is the existence of a masculine noun that can't have come from anything else than this allusive verb. This noun is מרבק (marbeq), meaning stall, and it's used a few times in the Bible, for instance in Amos 6:4 and Malachi 4:2.