'unmethodicalness' originates from English; it is formed by the negative prefix 'un-' (meaning 'not'), the adjective 'methodical' (from 'method' + suffix '-ical'), and the nominalizing suffix '-ness' (forming a noun).

'method' comes from Greek 'methodos' (meaning 'pursuit, way, method'), via Latin 'methodus' into Middle English as 'method'; 'methodical' was formed in English from 'method' + '-ical', and 'unmethodicalness' was later formed by adding 'un-' and '-ness' to 'methodical'.

The formation is a straightforward negative nominalization: it has meant 'the state of not being methodical' since it was coined and retains that meaning.