A phrase used out of anger, usually in place of "son of a bitch." As it contains no curse words, it's okay to say in public.
Teacher: So don't forget class, your essays are due tomorrow.
Student: Son of a dick! I totally forgot about that.
Teacher: Excuse me! What did you say?
Student: It's okay, I didn't say son of a bitch
, I said son of a dick. That's clean.
Teacher: Oh, I see. Never mind.