'irritatingness' originates from English, specifically formed by adding the suffix '-ness' to the adjective 'irritating'. 'Irritating' comes from the verb 'irritate', which ultimately derives from Latin 'irritare', where 'in-' meant 'on/into' and the root meant 'to excite or provoke'.

'irritatingness' developed from the modern English adjective 'irritating' (itself from the verb 'irritate'), with the nominalizing suffix '-ness' producing the noun; 'irritate' entered English via Late Latin 'irritare' and Middle English forms influenced by that Latin term.

Originally the Latin root was used in senses of 'to excite' or 'to provoke'; over time in English the verb shifted toward 'to annoy or make impatient', and the noun form now denotes the degree or state of causing annoyance.