"Bullshit" is an idiom, meaning it's literal translation doesn't mean the same as what people who speak English use the word for. To call something "bullshit" means that it's invalid, unworthy, untrue, etc. "His excuse that his dog ate his homework is bullshit." "I was fired from my job because I was 5 minutes late one time--that's bullshit." Keep in mind that "bullshit" would be classified as cussing, so I wouldn't use that when speaking with your grandmother!

Not just grandmother, even parents o.O lol.....

No it doesn't mean Duh which is an expression of surprise.

bullshit = complete nonsense, complete rubbish