British UK slang meaning "Calm down." or "Chill out." Comes from the word "hype" meaning hyperactive, and the verb "to jam", meaning to relax, to chill, or to hang out.
Referring to the ultra chill that some people just need to take. Like calm your life but better because one needs to 'jam' aka stop fretting! Letting things take their toll...not making plans but seeing how things go...
Saarah: 'at precisely what time should i meet you?'