'flat-sided' originates from English, specifically the combination of 'flat' + 'side' with the adjectival suffix '-ed', where 'flat' meant 'broad, level' and 'side' meant 'flank or lateral surface'.

'flat' comes via Old English/Old Norse (Old Norse 'flatr') meaning 'level, broad', and 'side' comes from Old English 'side' meaning 'flank'; the compound formation with '-ed' (adjectival) developed in Modern English to describe having the quality of those words, producing 'flat-sided'.

Initially the elements referred separately to a 'level/broad' surface and a 'side/flank', and over time the compound came to mean 'having flat sides' as a single descriptive adjective.