1. Crazy; insane; mad.
2. Crazy person; a lunatic.
From Hebrew via Yiddish meshugener, from meshuge (crazy) + -er (masculine singular person suffix)
Often pronounced without final -r, influenced by both Yiddish gender inflection and non-rhotic American dialectal pronunciation.
I was sitting there eating my breakfast when this meshuggener burst in the front door of the restaurant and tried to take us all hostage.