The building blocks of English

Affixes

-o

Marking informally shortened or slang nouns.

Perhaps from the interjection oh!, or the use of ‑o in ballads to terminate lines; its use has been reinforced by shortened forms ending in the linking vowel ‑o‑.

Though a wide variety of nouns in English end in ‑o, this suffix occurs only in words that have been formed from other native words in one of two specific ways. One method is to informally abbreviate a longer term, of which a few examples out of many are ammo, condo, hippo, limo, and photo. Others are based on an adjective or noun, to which the suffix is added to create a colloquial or slangy term, which is often—but by no means always—derogatory: beano (from beanfeast), boyo, cheapo, kiddo, pervo (from pervert), pinko, righto, sicko, weirdo, wino.