Incredibly deep anal penetration, typically performed by a man with an abnormally long penis that is able to stimulate the woman's (or man's) jejunum in the small intestine.
Man: Hey baby, you up for some jejunumal sex??
Woman: Oh hell yeah baby, I love feeling you that deep in my ass ;)
The worst word in the English language, jejune just bleeds pomposity and snobbery. If you seriously use jejune in a sentence you a) are automatically an asshole and b) should be slapped across the mouth.
"I detected a jejune air that had not irked me before."- Brideshead Revisited