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Comes from a way of describing hot dogs on a bun. Hotdogs are pretty tasty and we all know eating tasty things are fun to do. Thus you get fun on a bun. Generally used as a compliment.
Example 1. This hot dog is fun on a bun!

Example 2. That mime is sure fun on a bun!
by Fun on a bun July 24, 2006
meaning its going to be super fun

it cant be boring

its like your eating fun
ilove it at her house its fun on a bun
by Veggygirl714 August 20, 2009
1.) When something is really fun.
2.) Used in expression of joy about a sandwich.
1.) MAN, last night, we went down to the bar... it was fun-on-a-bun!
2.) Subway is always fun-on-a-bun!
by BitterPoetMadman December 15, 2006