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An adjective describing a relationship in which two close friends of the opposite sex are infinitely close to hooking up or having a formal relationship, but never quite do.

Derived from the mathematical concept of an asymptote, a curve coming infinitely close to a line, but never quite reaching it
"Come on, I saw Kristie looking at you last night at the party. Tell me you two finally hooked up"
"Nah, it just didn't feel right. I'm starting to think that our relationship is doomed to be ass-symptotal."
by Boris Arkadyevich January 28, 2008