flirt sink
a person with whom you flirt with primarily as a way to avoid over-flirtation with other people; a person doomed to the liminal space between friend zone and fwb for eternity

noun; adjective; stemming from the term heat sink, referring to an object used to dissipate excessive heat
Who's this bitty you're talking to, broseph? Anything there?

- Ah, nah homie. She just some flirt sink. She don't even live here.

I thought that he knew I just used him as a flirt sink, and now he wants to like talk about real life stuff. Ew!