BSS? What does that mean? I've just been calling it "poaching" this entire time.
BSS or "
Boku ga
Saki ni
Suki Datta" (I loved him/her first) is what the Japanese call the NTR-like scenario where the "partner" being "stolen" from the protagonist is not in any romantic relationship with them (and therefore isn't actually their partner). Because someone who was never yours technically can not be "stolen" from you, they call such scenarios BSS instead of NTR.
Childhood friends where neither side confesses but one gets taken away by a 3rd party through sex is a common BSS scenario. Them having feelings for each other doesn't matter much when it's unexpressed (so the "love" is technically only still in their heads, to others they're not in a relationship and the "partner" is "fair game").
Spineless MC: "Why did you take my childhood friend away from me!?"
Villain/rival: 'You weren't even dating her! What are you to her anyway?'
Spineless MC: "...But I loved her first before you did!"
Villain/rival: 'You loved her even though you did nothing for x years?'
In English, one could just call it plain ol' "jealousy". The funny thing about this is that the villain lied to the heroine, so she too gets a preview of BSS, thinking some other girl took her junior away. The junior was saved from experiencing actual BSS.