suffix, forming advs. The earliest instance is endlong, from ON. endlang-r adj., ‘extending from end to end,’ ‘the whole length of.’ The word is properly a compound of LONG a.; but in Eng. it was principally used as adv., and developed the sense ‘end-wise,’ ‘end foremost,’ so that it became parallel in meaning to words like sideling, headling, backling. The ending -long thus came to be regarded as a variant of -LING suffix2. Hence, on the one hand, the occasional 14th-c. form endelyng for endlong, and, on the other hand, the substitution of headlong(s, sidelong(s, flatlong(s for the earlier headling(s, sideling(s, flatling(s.
