Don't Give A Fuck Mode (DGAF) is a temporary (or permanent for some people) period of time when a person chooses to "not give a fuck" and do whatever he/she pleases without worrying about the consequences.
John: I heard Zach drank a bottle of whiskey, crashed his car, punched a cop, slept with two fat chicks, and asked his sister to marry him last night.
Chris: Yeah man, he definitely was in Don't Give a Fuck Mode... he probably had a rough morning.
Cleaning the slate is the act of getting so drunk that you wipe your brain clean of any recent information that you may have gathered. Slate cleanings are popular after big tests, projects, deadlines, or break-ups.
Josh had a really rough finals week, so he planned on cleaning the slate over the weekend to free up more space in his brain.