limb [OE] There was no ‘b’ in limb until the 16th century: the earlier form was lim. Old English thuma similarly could be thum or *thumb in Middle English. Words such as Old English comb and dumb that had always ended in -b may have influenced the new spellings. The limb in out on a limb [L19th] is the branch of a tree, a sense of the same word. The image conjured up is of someone clinging precariously to the end of a projecting branch, with nothing or no one to assist them in their difficult situation.