'one-sidedness' originates from English, specifically from the adjective 'one-sided' plus the noun-forming suffix '-ness'; within that, 'one' ultimately comes from Old English 'ān' meaning 'one', and 'side' from Old English 'sīde' meaning 'side, flank, or aspect'.

'one-sidedness' changed from the compound adjective 'one-sided' (formed in Early Modern English from 'one' + past-participial-like '-sided') with the addition of the noun suffix '-ness' (from Old English '-nes(s)a'), eventually becoming the modern noun 'one-sidedness'.

Initially it referred most literally to 'having or pertaining to one side'; over time the sense broadened to include 'imbalance' and the evaluative sense of 'partiality' or 'bias', which is common today.