A Figure of Speech; when the "bitch" of your group of friends decides to do something bold and unexpected of them, you can simply remind them that Mr. Happy won't be pleased.
Mr. Happy can be any form of a terrifying creature, but in this case, he is a serpent lodged deep inside a seemingly nice womans vagina. When that woman becomes enraged or disgruntled, she sends Mr. Happy to do her dirty work.
Forms of attack include severe pestering, extreme discomfort, headaches, the ability to put one in a bad mood, extreme hair growth on thighs, shoulders, and face, and on occasion; death.
"I'm going to stay up until 6 p.m. today!"
--"Peter I wouldn't want to make Mr. Happy upset."