1.
If you describe something or someone as jejune, you are criticizing them for being very simple and unsophisticated. (FORMAL)
They were of great service in correcting my jejune generalizations.
ADJ [disapproval]
2.
If you describe something or someone as jejune, you mean they are dull and boring. (OLD-FASHIONED)
We knew we were in for a pretty long, jejune evening.
ADJ