stfu

"stfu" basically stands for "shut the fuck up. sometimes its sarcastic, and sometimes the person wants to kill you.

so when you got a text back from your friend saying stfu, they might not be your friend anymore, or they are joking.
But it basically means shut up...

Sarcastic:
Josh: where do penguins come from?

Jane: stfu

Serious
Grandma: Heres your food!

Kid: i wanted chips damnit!

Grandma: stfu