1. 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 ).
2. a suffix occurring in colloquial noun or adjective derivatives, usually grammatically isolated, as in address: cheerio; kiddo; neato; righto.
[ perh. 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 orig. ]