What does it mean when people say this? mainly on twitter..?

[Mitsuki]

'haseeb' is wrong. 'I can't" does not mean that. It's real meaning is when something is supposed to be so funny/ strange/ weird that the speaker cannot help laughing. 'I can't' was once derived from 'I can't breathe' (because of laughing).

For example: If you saw someone wearing really bad trousers, you might say, 'I cant!' to your friend, meaning "I can't believe it, I can't cope, I'm going to laugh. I just can't look."

Or another example would be: If your friend just told you a joke and you were crying from laughter. 'I can't' would take the meaning of 'I can't breathe.'

It's mostly used by the younger members of the online community. I hope this helps you!