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1.
(v.) to add additional gasoline to the gas tank even though the gas tank container is full. People do it to get the most bang for their buck, to have more gas in their car. This action can be dangerous and it is prohibited by most gas stations. Most gas station attendants do it anyway to sell more petroleum.
"No topping Off at my station" says the automotive saftey expert.
by T-Dog Jenkins May 01, 2005
 
2.
Having one person after another poop in a toilet till the point where the poop mountain peaks just above the rim of the toilet.

Topping you off is referred to as topping off someone else's dookie.

The expression "top you off" can also be said to mean something of the same, metaphorically, as a joke.
"The kids at the school topped off the toilet in the band-wing so high that the toilet wouldn't flush. Having never experienced anything like this, the janitor had to call for back up to scoop out the ever-ending pile of shit. This phenomenon created the art of topping off."
by Invented at Avon Grove High July 29, 2009
 
3.
pooping in someone else's butt
"yo. what are you doing?"
"im toppin my girl's dog off. lemme call you back"
"just text me."

"oh yeah, top me off baby"

"you better shut up or im gonna knock you out and top you off"

the act can be represented as itself in the present tense; topping off. booyah grammar kids. im gonna top y'all off
by dmoney,son December 14, 2007