Among the more curious classes of slang words is that of terms ending in the letter o, the topic of this post.
Several categories exist in which informal words end in o. Among the oldest are those consisting of words to which an extraneous o has been added, such as cheerio (from cheer or cheery), which in British English is used as a greeting or a farewell, and boyo, an extension of boy, just as kiddo is derived from kid.
Sometimes, a word ending in o is simply a truncation of a word in which o is naturally the last letter of the abbreviation, such as condo, for condominium, which originally meant “joint rule or sovereignty” but in the mid-twentieth century came to refer to a privately owned apartment. (The word to which the element con-, meaning “with,” has been attached is cognate with domain.) Another example is demo, short for demonstration, in the sense of a presentation of a product or a rough version of a music recording.
Sometimes the o is attached to a truncated word after the first syllable regardless of which vowel follows in the full form of the word, as in aggro, for aggressive (generally to describe a hostile person or hostile behavior); ammo, for ammunition; combo, for combination (usually in the sense of a small ensemble of musicians, especially those who play jazz); convo, for conversation; and journo, for journalist.
Australia is fertile ground for word formation of this type (as well as other diminutive forms, such as applying -ie, as in alkie, for alcoholic, and barbie, for barbecue). Some more obscure abbreviations from Australia include arvo, for afternoon; doggo (in the expression “lie doggo,” meaning “lay low”); and muso, for musician.
One slightly truncated word in American English is rando, from random; several decades ago, the latter word was first employed as a noun to describe a peculiar person, and from there it acquired the sense of someone who is sketchy or undesirable. Rando inherited that connotation, usually in the context of a stranger who unexpectedly tries to make contact with another person, either in person or through social media. An older, synonymic term is weirdo, which sports an o attached to weird. On this model was beardo formed to refer to someone sporting a trendy-looking full, bushy beard, especially one who might also be tagged a rando or a weirdo.
Occasionally, a word ending in o is adopted from another language. In the case of mondo, which denotes excessiveness or outrageousness, it derives ultimately from the Italian word for world; the context is Mondo Cane, the title of an Italian documentary about human eccentricity. (The loose English translation of the title is A Dog’s Life; cane is cognate with canine.)