Well...I have ALL the squares filled in and they all "add up" correctly but my solution is WRONG. So now I have to figure out where the heck I went wrong! May take awhile!!
Some people cannot cook. They burn breakfast so bad that you lose your lunch preemptively. When they cook dinner, you can only think of dessert-ing. Even the Rats Won't Touch It. Sometimes, if you're lucky, they can cook something that can be charitably described as "food."
Sometimes their food merely tastes bad. However, it has also been known to cause nausea, vomiting or hospitalization. If you're particularly unlucky, it can cause, well, just about anything. If dinner eats you, you're probably in a lethal chef's kitchen.
As you may have guessed, this is usually a comedy trope. In eastern media, most examples are female (emphasizing that they fail at an aspect of traditional femininity, or femininity in general, or that they're something of an Action Girl), but there are a ton of male examples in western media, probably tying in to the assumption that Men Can't Keep House and Dads Can't Cook.
There are a number of common variants. Sometimes someone is forced to stomach the lethal chef's food to avoid hurting their feelings. Sometimes they refuse to admit that they can't cook, despite mountains of evidence. And sometimes, if you're particularly lucky, their food merely looks bad, and tastes perfectly fine.
Lethal Chefs may be employees of a Lethal Eatery, and quite often can be found fixing up a stew of Mystery Meat. Most Camp Cooks fit this category. (Truth in Television for them, as their budgets and ingredients are limited.)
Compare the Cordon Bleugh Chef who can cook just fine, but often gets too creative, the One-Note Cook who can ace one dish, but otherwise falls into this trope, and the Evil Chef who just likes to see you suffer. See also Lethally Stupid when the character is so dumb it would be dangerous to let him cook. Contrast the Supreme Chef, who is the exact opposite of this trope, the Angry Chef who has nothing but contempt for his customers and peers, and the Chef of Iron, who can be either bad or good but is lethal outside of their cooking.