This was uttered by a driver who was being tailgated. It is a congruent conflation of “calm down” and “cool your jets”, both meaning to tell someone to relax or be less intense. In researching this one, I found out that “calm your tits” is another expression meaning to relax or be less intense, so perhaps the speaker was thinking of this one as well. Anyone know if that is an American or British expression? A big thanks to Martin Pietrucha who said this one and realized he had malaphored.