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-o

  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.
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Origin of -o

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
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for -o

-o

suffix
  1. forming informal and slang variants and abbreviations, esp of nounswino; lie doggo; Jacko
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Word Origin

probably special use of oh
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012