Caveman Mode is the act of doing something so stupid, your brain must have reverted to that of a caveman. This holds the same meaning and use as brain fart does.
Person 1: What were you thinking?!?!
Person 2: My brain went caveman mode
When you're just too horny and can't really think about what you're doing.
When you're in caveman mode, it's important to ejaculate as soon as possible in order to reach post-nut clarity, or it could lead to dire consequences.
Don't stay in caveman mode too long, we don't want you going to jail for cumming in that girl's coffee.