'annoyingness' is formed in Modern English by adding the adjective-forming suffix '-ing' and the noun-forming suffix '-ness' to the verb 'annoy'.

'annoy' comes from Old French (variants like 'anoier'/'enoiier'), which entered Middle English as forms such as 'annoyen' and developed into modern English 'annoy'. The adjective 'annoying' (present participle) and the nominalizing suffix '-ness' later produced derivatives like 'annoyingness'.

Initially the ancestor words meant 'to cause trouble, vex, or cause frustration'; over time 'annoy' narrowed to 'irritate' or 'bother', and 'annoyingness' now denotes the degree or quality of being irritating.