Etymology
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-eroo 
"factitious slang suffix" (OED), sometimes affectionate, forming nouns indicating "a humorous or remarkable instance" of what is indicated, in use by 1940s, perhaps from buckaroo. An earlier suffix in a similar sense is -erino (after 1900), apparently from -er + Italian suffix -ino.
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-ent 

word-forming element making adjectives from nouns or verbs, from French -ent and directly from Latin -entem (nominative -ens), present-participle ending of verbs in -ere/-ire. Old French changed it in many words to -ant, but after c. 1500 some of these in English were changed back to what was supposed to be correct Latin. See -ant.

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-ity 

word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).

Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]
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