'anomalousness' ultimately originates from Greek, specifically the word 'anōmalos' (and related noun 'anōmalia'), where the prefix 'a-' meant 'not' and 'homalos' meant 'even' or 'level'.

'anōmalos' (Greek) and 'anōmalia' (Greek noun) passed into Late Latin and Old French forms (e.g. Latin 'anomalus'/'anomalia', French 'anomalie'), and the adjective 'anomalous' and noun forms were later formed in English; 'anomalousness' is an English derivative formed from 'anomalous' + '-ness'.

Initially it meant 'uneven' or 'not even/regular' in Greek; over time the sense generalized to 'deviating from the norm' and this remains the central meaning in modern English.