'perilousness' originates from English, specifically formed from the adjective 'perilous' plus the suffix '-ness'; 'perilous' ultimately traces to Late Latin 'periculosus', where 'periculum' meant 'danger'.

'perilousness' changed from Middle English forms such as 'perilousnesse' (influenced by Old French 'perileus' and Latin 'periculosus') and eventually became the modern English word 'perilousness'.

Initially the root 'periculum' meant 'danger' in Latin; over time the derived English noun came to mean 'the quality or state of being dangerous' (i.e., 'perilousness').