Biography
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.
At worst, if a dish looks abominable beyond comprehension, it may be censored for the sake of the audience.
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 increasingly used purely for the Moe factor), and in Japan is known as メシマズ or "meshimazu", 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. Even the most chauvinistic of men with a "Stay in the Kitchen" attitude towards women wants the exceptional lethal chef to Stay out of the Kitchen.
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.
The primary reasons for their failures can vary from case to case, but common culprits tend to be either too klutzy, or too experimental. The first case is prone to burning things, mixing the wrong (or non-edible) ingredients, forgetting important steps, or even neglecting the basic "wash your hands" before starting. The other type seems to function on the idea that using a cookbook or recipe is cheating or that they're otherwise nonexistent, and instead attempt to make something from scratch with whatever they think works best, failing to realize that not all foods are meant to be mixed together.
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 makes meals like this on purpose just to see you suffer. See also Lethally Stupid when the character is so dumb that 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 will probably reserve his nastiest profanities for chefs like this), and the Chef of Iron, who can be either bad or good, but is lethal outside of their cooking. Not to be confused with actual homicide by cooking, which is Cooked to Death, nor using particular cooking utensils as weapons, which is Frying Pan of Doom.