You probably thought that you'd break my heart (Ooooohoooh)
You probably thought that you'd make me cry

from The Weeknd – The Knowing Lyrics on Rap Genius

Meaning

She thought he would be heart broken and crying because of her cheating… not the case.

It’s also a defense mechanism — that he is pretending like he is OK, but he really is heartbroken and hurt. His revenge tactics helped him, but didn’t heal him. So even though he says it’s OK, it really isn’t. he’s hiding behind the drugs.

Also, In the music video the girl rips his heart out, leaving him heartless. This could also mean instead of him just being temporarily heartbroken (like she expected), he’s now heartless.

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