'athrong' originates from Old English elements: the prefix 'a-' (from Old English 'on' meaning 'in' or 'into') combined with 'throng' from Old English 'þrong' meaning 'crowd'.

'athrong' appeared in Middle English as a- + 'throng' (from Old English 'þrong'); the productive prefix 'a-' formed adverbial/adjectival compounds (compare a-field, abed), and 'throng' continued as the noun/verb meaning 'a crowd' before yielding the compound form.

Initially it meant 'in a crowd' or 'together in a throng'; this core sense has been retained, though the word is now rare/archaic.