When one has a unexpected erection and is forced to walk into public and hide the erection in his waistband.
Tom woke up with a boner with a knock at the door so he performed the faithful buckle-tuck to stealthfully have a normal conversation with a raging hard-on.
When a man gets an erection in an uncomfortable public area and must tuck his penis into the buckle of his belt as to not allow others to see his excitement.
"hey dude look at those hot chicks"
(your wiener starts to rise, and you realize your grandma is across the room)
"dude I'm buckletuckin'!"
Similar to a hucklebuck, with the exception that the bucklebuck is even more exaggerated to the point where the girls legs are behind her head and contorted as such that she resembles a pretzel, or belt buckle.
She threw her legs behind her head for a hucklebuck fuck, but I bent her ass into a pretzel for a full on BUCKLEBUCK!