This is a Buggs Bunny original, implying the object individual to whom this utterance is directed must make a real, conscious effort to relax.
The Tasmanian Devil is fast on the trail of Marvin the Martian, who is sweating blood to escape. Buggs pulls him aside by the scruff of the neck.
what's up doc?
you's gotta unlax.
... it's only a cartoon.
and a furry bunny hand reaches outside of the computer TV monitor and punches "pause" stopping the dastardly demon in his tracks, while Buggs and Marvin have a casual chat.