a suffix occurring as the final element in informal shortenings of nouns (ammo; combo; condo; limo; promo); -o, also forms nouns, usually derogatory, for persons or things exemplifying or associated with that specified by the base noun or adjective (cheapo; pinko; sicko; weirdo; wino).
a suffix occurring in colloquial noun or adjective derivatives, usually grammatically isolated, as in address:
cheerio; kiddo; neato; righto.
perhaps orig. the interjection O, appended to words as in def 2; as a derivational suffix reinforced by clipped forms of words with -o- as a linking element (e.g., photo, stereo), by Rom nouns ending in o, and by personal nouns such as bimbo and bozo, of obscure origin