'single-sidedly' originates from Modern English, formed by adding the adverbial suffix '-ly' to the adjective 'single-sided' (itself a compound of 'single' + 'sided'). 'single' ultimately comes from Latin 'singulus' meaning 'one' and 'side' comes from Old English 'sīd' meaning 'side, flank'. The suffix '-ly' derives from Old English '-lic' meaning 'like'.

'single-sidedly' developed from the adjective 'single-sided' (Modern English). 'single-sided' arose by compounding 'single' (from Latin 'singulus' via Old French/Old English developments) with 'sided' (from Old English 'sīd' + adjectival '-ed'), and later the productive adverbial suffix '-ly' was attached to form the adverb.

Initially the component words simply meant 'single' (=one) and 'side' (=flank or aspect); over time the compound 'single-sided' came to mean 'one-sided' or 'biased', and 'single-sidedly' now means 'in a one-sided or biased manner.'