`屁话` is extremely common in informal, everyday speech and online. Its usage is highly dependent on context and relationship.
In Arguments: This is its most common and aggressive use. Saying `你说的都是屁话!` (“Everything you're saying is bullshit!”) is a way to completely invalidate the other person's argument.
Among Friends: It can be used in a teasing, playful manner. If a friend makes a ridiculous exaggeration, you might laugh and say `屁话!` to call them out. The tone of voice is key here.
On Social Media: `屁话` is rampant in online comment sections, used by netizens to express disgust or disbelief towards news articles, official statements, or other users' posts.
Self-Deprecation: A person might say `哎,我刚才说的都是屁话,别当真` (“Ah, what I just said was all crap, don't take it seriously”) to humorously downplay their own opinion or to backtrack from a statement.
Formality: Strictly informal. Never use `屁话` in a professional, academic, or formal setting, or when speaking to elders or superiors. It is considered very rude and disrespectful.