Fig newtons are ambiguous, they're not biscuits/
cookies and they're not fruit, thus:
1) A fig
newton is a noun that defines a relationship that is more than just being "friends with benefits." You have sex but you actually have feelings for each other, you are emotionally dependent on each other but it'
s not (yet) love, and for some reason (i.e geographical distance, availability, commitment issues, etc), you two have chosen to not be in an exclusive relationship.
2) A fig newton is for a relationship you have with someone that you cannot categorize as "just friends," "fuck buddies," "friends with benefits, or "boyfriend/
girlfriend."
"Hey, so what's going on with that
guy that you were seeing?"
"
Oh, he moved to NYC. We still have feelings for each other and have sex when we see each other though. But we're not in a relationship or anything"
"What do you call those things?"
"
Oh, he's my fig newton (n)." or "We're fig newtons (pl)"