An expression to say that someone is talking without thinking

What idiom can be used define a the situation where someone is telling something without thinking? Possibly a slang definition.

Is "Don't say bullshit" a possible answer?

Update: I fount the answer, "Cut the crap!", of course the answers below are correct too.

Solution 1:

blurt - to say (something) suddenly and without thinking about how people will react

Although the Merriam-Webster definition doesn't actually mention it, blurt out is a common form.

OP's specific context ("You say XXX") doesn't match any standard English expression. There are various slangy versions of "You talk / You're talking XXX" (where XXX may be rubbish, rot, crap, [bull]shit, bollocks, etc.). But they all focus on the idea that what you're saying is worthless, rather than that you're speaking without thinking.

Possibly "You're talking out of your ass" might come close to "without thinking", since to some extent it's modelled on "You're talking off the top of your head".

Solution 2:

Consider the expression "say something offhandedly (or in an offhanded/offhand manner)."

The idioms "off the top of one's head" and "off the cuff" might also do the job for what is meant in the OP.

The speech was entirely off the cuff, and obviously straight from the heart.

Perhaps the things that are said off the top of one's head are truths intuited before blunt proof of the obvious is apparent.

I once said offhandedly to a coworker that had just been run over by a truck, "Wow! You look like shit!" It was meant to be disarming but it came off rather crass.