The One With Joey's Fridge - S6-E19
Question: In this episode there is a lot of reference to what seems to be some American custom I don't get. Monica says to tell Elizabeth that she "wants her cookies early this year" and later Ross orders "one box of thin mints". What is this all about?
Answer: This would be a reference to the Girl Scout cookie selling drive each year, to raise money for the organization. Girl Scouts are typically (not always) known as being younger children. So Monica is making fun of how young Elizabeth is by referring to her as a Girl Scout. The Thin Mint is one of the perennial best-sellers, chocolate-mint wafers coated in a layer of dark chocolate - Ross is saying it so the professor that walks in on him and Elizabeth will think he's just buying cookies.
Rooster of Doom