"long and loose-jointed," by 1812, from Scottish and Northern English gang (v.) "to walk, go," which is a survival of Old English gangan, which is related to gang, n.. The form of the word is that of a present-participle adjective from a frequentative verb (as in fondling, trampling), but no intermediate forms are known. The sense extension would seem to be via some notion involving looseness in walking.