'scariness' originates in Modern English from the adjective 'scary' plus the suffix '-ness', where 'scary' comes from the verb 'scare'.

'scary' developed from the verb 'scare' (attested from the mid 16th century). 'Scare' itself is probably of Scandinavian origin (compare similar Old Norse or Middle Scandinavian forms) and entered English earlier; the adjective 'scary' appeared later, and the noun 'scariness' was formed by adding the productive noun suffix '-ness'.

Originally related to the verb meaning 'to frighten', the derived adjective and then noun shifted to denote the quality or degree of producing fear; 'scariness' now specifically denotes the quality or extent of being frightening.