Not very much in the way of intelligence. She seems nice enough, but I get the sense that there's nothing upstairs. Jake's boyfriend is incredibly attractive, but he has nothing upstairs from what I can tell.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Fig. no brains; stupid. Tom is sort of stupid. You know—nothing upstairs. I know what's wrong with you. Nothing upstairs.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
phr. no brains; stupid. Tom is sort of stupid acting. You know—nothing upstairs. I know what’s wrong with you. Nothing upstairs.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.