'side-to-side' is formed from the Old English word 'sīde' (modern English 'side') combined with the preposition 'to' (from Old English 'tō'), producing the phrase 'side to side' which later became hyphenated.

'side' originates from Old English 'sīde' meaning 'flank, border', and the preposition 'tō' gave the connective meaning 'toward' or 'to'; the repeated construction 'side to side' has been used in English for centuries and in modern usage it is often written as the hyphenated adjective/adverb 'side-to-side'.

Initially a simple combination meaning 'from one side to another', the expression has retained that basic sense but has also been generalized to describe lateral or sideways movements and arrangements in modern usage.