'unmetrical' originates from English, specifically the prefix 'un-' and the adjective 'metrical', where 'un-' meant 'not' and 'metrical' ultimately traces back to Greek 'metron' meaning 'measure' (via Latin/Old French developments).

'metrical' changed from Late Latin/Medieval Latin forms related to 'metrica' and Old French 'metrique' and eventually became the modern English 'metrical'; the negative prefix 'un-' was attached in English to form 'unmetrical'.

Initially it meant 'not measured' or 'not conforming to measure'; over time it has come to be used specifically for 'not conforming to a regular poetic meter' or 'lacking regular rhythm'.