How do you say what the heck are saying in Japanese?

There isn't a single perfect translation for "What the heck are you saying?" in Japanese, as the best way to say it depends on the context and your desired level of formality and intensity. Here are a few options, ranging from mild to strong:

Mild:

* 何言ってるの? (Nanitte iun no?) - This literally means "What are you saying?" and is a neutral way to express confusion or disbelief.

* どういうこと? (Dō iu koto?) - This means "What do you mean?" and is slightly more polite than "Nanitte iun no?"

* ちょっとよくわからないんだけど… (Chotto yoku wakaranai n da kedo…) - This means "I don't quite understand..." and is a more gentle way of saying you don't understand what someone is saying.

Stronger:

* 何言ってんの? (Nanitte n no?) - This is a more forceful version of "Nanitte iun no?" and implies frustration or annoyance.

* どういうこと言ってるの? (Dō iu koto itten no?) - This is a more direct and forceful version of "Dō iu koto?"

* 何を言っているんだ? (Nani o itte iru n da?) - This is a very direct and strong way of saying "What are you saying?" and is often used in a confrontational situation.

* 一体何を言っているんだ? (Ittai nani o itte iru n da?) - This adds an emphasis on "exactly" or "in the world" to the previous phrase and is even stronger.

More informal:

* え、まじ? (E, maji?) - This means "Really?" and is a casual way to express disbelief or shock.

* うっそーん! (Usso-n!) - This means "No way!" and is a playful way to express disbelief.

Important Notes:

* The tone of voice and body language can greatly affect how these phrases are interpreted.

* Using strong phrases like "一体何を言っているんだ?" can be seen as rude or aggressive, so use them with caution.

Remember to choose the appropriate phrase based on your relationship with the person you are speaking to and the context of the conversation.