'straightly' originates from Old English, built from the adjective 'straight' plus the adverbial suffix '-ly' (from Old English '-lic' meaning 'like' or 'having the nature of').

'straight' changed from Old English 'streht' or 'stræht' (from Proto-Germanic '*straihtaz'), later Middle English 'streit' or 'straight', and combined with the adverbial suffix to form 'straightly' in later English.

Initially it conveyed the sense 'rigidly or in a direct line'; over time it evolved to the broader adverbial sense 'in a straight or direct manner' and 'immediately' in some idiomatic uses.