When two single people spend as much time together as a pair in a relationship but aren't sexually involved with each other. The term applies best to opposite sex pairs or same sex pairs where at least one is homosexual. Especially applies to situations where one party wants the pair to be sexually involved.

As in half of an open faced sandwich waiting exposed for the other to be on top of it and for their meats and other juicy bits to be smooshed together and mixed.

Friendzone
Why don't Brett and Meg fuck already? They're always together

Haha, I know. He wants it but she just wants to be friends. They're in an open-face sandwich relationship
by Double-decker meatwhich February 9, 2018
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When both of your best friends are in relationships but you are still single.
I can't believe Jenny got a boyfriend and Hannah has one too they're my best friends and I'm still single! I hate being relationship sandwiched!
by Absterline June 24, 2014
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