It may also be used metaphorically, and mean that you had a bad sex experience in the morning, and it caused a bad attitude for the rest of the day.
Joe: Dude, whats with your pubes?
John: I didn't have enough time in the morning to trim them.
Joe: That sucks. I hate bad pube days. It looks like Donald Trump down there.
Boss: Alright guys. You've been by the watercooler for too long. Back to work!
Joe: Hey John. Did you send that fax yet.
John: SHUT THE FUCK UP. I'M ON IT!
Joe: What his problem?
Boss: He came too fast this morning and his wife got REALLY pissed. This is probably his most intense bad pube day I have ever seen.