我肏

Vulgar - strictly informal speech and the internet. 肏 is the 'proper' character but online you'll mostly see 我操, 我靠 (softer), or just 卧槽 (a homophonic dodge). Functions exactly like English 'holy shit' - surprise, frustration, awe - not a literal threat.

mistakes

Don't drop this in front of your boss, language partner's parents, or any TV camera. It's at the heaviest end of the surprise-interjection scale; 卧槽 is a much safer everyday substitute.