Clueless person. May be spaced out, oblivious. Common sense and/or common courtesy are alien to them. Applies to both someone who is unintentionally/passively irritating, and one who is deliberately so.
It's a silly put-down, so chances are after you use it your anger/annoyance will subside a little.
"You nimnoid, you swiped the last ice cream sandwich! You knew I wanted that!"
Someone cuts you off in traffic. You shout after them, "NIMNOID!"
An obnoxious person finally vacates your personal space. You mutter after them, "nimnoid..."
