Some 'ous' adjectives have the noun form 'ity', e.g. enormous > enormity, hilarious > hilarity.
Some have the noun form 'osity', e.g. monstrous > monstrosity, religious > religiosity.
Some have the noun form 'ousness', e.g. nervous > nervousness, outrageous > outrageousness.
'Ingenious' has the irregular noun form 'ingenuity' (the 'u' presumably arose through confusion with 'ingenuous').
Some 'ous' adjectives have alternative noun forms, e.g. callous > callousness or callosity.

I don't know why they vary in this way. Such words are mostly derived from Latin. Some may have been derived directly, and others via French; I don't know whether that accounts for any of the differences.

Last edited by A C Bowden; 07/15/2018 1:52 AM.