Oral sex performed on a man while he's asleep.
Ideally: while he's experiencing morning wood and resulting in his being roused from sleep.
See also: morning head.
Note: The "ZJ" version was made famous by the (excellent) show beerfest
A: My baby's momma gave me a ZJ this morning! :-D
Q: Did she swallow?
A: Fuck yeah she swallowed; She knows not to leave a big white stain on my sheets.
2)He looked half dead from some of that Northern Lights we got in BC. So, I just grabbed his tool and gave him a ZJ.