CTFD

CTFD is an extremely emphatic synonym for chill. Typically, people use this acronym when they think someone else is overreacting about something.

Of course, if someone is already upset, telling them to CTFD likely won't make things better. (Most people don't like being sweared at, it turns out.) So you may want to think twice before sending CTFD, and instead find another way to say "calm down."