This expression is used when a person does or tells you something you already know and thinks it is absolutely amazing.
Person 1: She got a hollywood wax for the first time.
Person 2: let me guess its the most exciting thing since sliced bread for her.
Another word for vagina
"Give me a piece of that finely sliced French bread"
a term coined by yours truly, refering to a traitor or similar (in reference to a disease you used to be able to get from eating rye bread called ergotism)
(in a game of among us)
Blue: WHY! FINE BROWN IS THE OTHER IMPOSTOR
Brown: You filthy slice of rye bread