It’s actually two words crammed into one.
“Jejune” means lacking substance, empty, either in the physical or emotional sense. A jejune body is devoid of nourishment and thin. An idea can be jejune if it’s boring, empty, shallow, or trite. “Hollow” is a good way to think about it. Insubstantial.
A jesuit is a member of a Catholic religious order.
The phrase “jejune jesuit” is taken from James Joyce’s Ulysses. I was probably 9 or 10 years old when I first tried to read that book. (I was a very pretentious child, but in my defense, it is on the second page.) Since I had no idea what either the word “jejune” or “jesuit” meant, its alliteration was the only thing that stood out to me. I pronounced it “jeh- JUNE jeh-SUIT” in my mind; I thought it sounded an awful lot like my name, Joseph.
So I stole that phrase for my online moniker, and I still use it around the net. Although I know what it means now, I still like it most as a sound.
And now that you know it’s not gibberish, hopefully you will remember it forever ;]