a one
"A one" is used as a shorter version for "one in a million" after someone's said something hillarious and/or classic.
Basically- it means they're pure legend :)!
Synonyms for a one
On this page, you will find 30 slang terms related to a one. Some of the top words include: monosyllable, mill, in my pants, Haplo, razor, and 25 more.