'inequilateral' originates from the Latin prefix 'in-' (meaning 'not') combined with 'equilateral', which derives from Latin elements 'aequus' ('equal') + 'latus' ('side').

'equilateral' comes into English via Medieval/Modern Latin (aequilateralis) and Old French influences as 'equilateral'; the negative form 'inequilateral' was formed in English by adding the Latin-derived prefix 'in-' to 'equilateral'.

Initially and historically the components meant 'not' + 'equal-sided'; the compound has retained the meaning 'not equilateral; having unequal sides' in modern usage.