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1.
a guy who you would consider more than a hookup, but not quite a boyfriend
After several months of casual sex followed by some serious dates, Jill saw Mark as her boy-thing, but she wasn't ready to make it official.
by Argonfire876 June 29, 2008
 
2.
you're sleeping with them, and you're not sleeping with anyone else, but they're not your boyfriend. You want to see where things go, without committing to a relationship. Somewhere between casual and serious.
Maru: So, is he your boyfriend yet?

Nancy: No, he's my boything.
by Mexi-ginge July 17, 2011