A threat for surprise retribution at a later date for an earlier transgression that usually results in the instigator missing teeth. Similar threats include "watch your back" or "you'll get yours". The notion of the saying is that retribution will happen suddenly when the instigator least expects it, such as sleeping and their teeth are placed under their pillow. The saying is otherwise used broadly to mean any level of revenge.
Someone does another person wrong or victimizes them, the victim may say "Watch yourself! You're gonna get a visit from the tooth fairy!"
Or used to dispel concern by others that retribution is going to be made "They're gonna see the tooth fairy, don't worry about it."
The act of defecating into the pillow sheet of another person, due to extreme aggression, hostility, rage, or an attempt at a very funny practical joke.
Ryansnuck into Tyler's room and gave him the dirty tooth fairy.
Fogey/fogy /fougi/ sl. (early 18C+, orig. Scot) old-fashioned, stuck-in-the mud.
Person with old fashioned ideas which he is unwilling to change: Come to the disco and stop being such an old fogey!
You think me an old fogeyand an old tory, his thoughtful voice said. I saw three generations since O’Connel’s time. I remember the famine. Do you know that the orange lodges agitated for repeal of the union twenty years before O’Connel did or before the prelates of your communion denounced him as a demagogue? You fenians forget some things. (James Joyce, Ulysses. Penguin Books,1992. p. 38)