It is a term for a day of adjustment.It is a rule where when you change something, it takes one day to adjust. Like when you buy a new tube of toothpaste and use it for the first time you find it awful but the next time you totally forget about it. Or when you drive on the other side of the road in another country , it takes one day to adjust to the change.
"Oh, it's like the toothpaste rule"

"It's exactly like the toothpaste rule , I'm gonna call of the toothpaste rule "

"No,no Burnie, it's the toothpaste rule. Anything that takes a day now — toothpaste rule."

"I'll see you next toothpaste rule "
by Piewarrior April 7, 2016
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This rule describes the phenomenon where when you get new toothpaste it'll only taste weird for a day. This rule can be applied to any event that becomes common place after about a day.
"Dude, my new toothpaste tastes horrible!"
"Don't worry about it, remember the toothpaste rule."

This was a term coined by Gavin Free from Roosterteeth Podcast #370.
by actuallynoplease June 6, 2016
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