It means “I fuck with you” It’s to say, that a person thinks you’re cool. Whoever said this would hang out with you and likes your vibe.
Can I come? I fw you, hop in the car.
When a shorty likes you very much
Nigga: do you fw me?
Shorty: I fw you heavy
To like/appreciate/vibe with someone to a great extent OR to fuck with someone heavily
“Yo dude I listened to your music on Spotify; I fw you obesely”