'asweat' originates from Old English elements: the prefix 'a-' (from Old English 'on/ā', meaning 'in' or 'on') combined with the noun 'sweat' (Old English 'swāt' / 'swǣte').

'asweat' appeared in Middle English in forms such as 'a-swāt' or 'a-sweat' and was written both with and without a hyphen before settling into the form 'asweat' in some texts; it has appeared chiefly in older and literary usage.

Initially it meant 'in a state of sweat' (literally 'in sweat'); over time the literal meaning has remained but the word has become archaic or literary, with modern English favoring phrases like 'sweaty' or 'in a sweat'.