(as) plain as porridge

1. Very obvious or noticeable; very easy to understand. It's plain as porridge that they like each other— they've been flirting all night! In the end, the solution was as plain as porridge. These directions are plain as porridge, so you shouldn't have any questions.

2. Very homely, unattractive, or dull. If you don't put on a little bit of make-up, this guy will think you're plain as porridge, which is kind of the opposite of what we're going for. Honestly, he looks plain as porridge compared to other guys I've dated. But he's smart and sweet, and that's all that matters to me. Who cares if she's plain as porridge? Her father's a billionaire—do you know how much money you could inherit one day?