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-ismo Suffix forming abstract nouns from verbs.
Borrowed from Greek -ισμός (-ismós) 'id.' Rebracketing of the the verb-forming suffix -ῐ́ζ- (-idz-) and the older noun-forming suffix -μός (-mós).

Suffix -ῐ́ζ- is from the verbal suffix *-idi̯eti in Proto-Indo-European. From *-id- and the *-i̯eti, a suffix that turned transitive verbs into intransitive imperfective verbs.

Suffix *-i̯eti became the -io-stem in Latin, appearing in third and fourth conjugation verbs (e.g., Latin capio "I capture" ); and, for the most part, has not survived in any obvious way in Spanish. Latin -io has been leveled to -o (capio is now cabo), conforming with other Spanish first-person singular endings.

The noun-forming suffix -μός is from Proto-Indo-European *-mo-s (whence -mo).