'dangerousness' originates from Modern English, specifically formed from the adjective 'dangerous' plus the nominalizing suffix '-ness'.

'dangerous' developed from the adjective built on the noun 'danger'. 'Danger' entered Middle English from Old French 'dangier' (later 'danger'), and the English adjective 'dangerous' was formed by adding the suffix '-ous'; later the suffix '-ness' produced 'dangerousness'.

Initially related to the notion of 'danger' (exposure to harm or loss), the compound word came to denote specifically the 'state or degree of being dangerous' in modern usage.