sell someone short

sell (someone, something, or oneself) short

To undervalue someone, something, or oneself; to underestimate or underappreciate the good qualities of someone, something, or oneself. Don't sell yourself short—your writing is really excellent! I know you don't like their pitcher, but don't sell him short; he's inarguably one of the best in the league. I think they're selling this short. It's actually an incredibly good deal!
See also: sell, short

sell someone short

If someone sells you short, they do not provide you with everything that they should. Students can't cope with too much complexity. But, on the other hand, if the tutor makes things too simple, that's selling them short too. If a film is worth showing, it is worth showing as the full work it was intended to be. Anything less is selling us short. Note: The reference here is to someone being cheated by being given less of something than they have paid for.
See also: sell, short