What's Under Double D's Hat?

There is nothing under Edd's hat but it was given to him by his mother for comfort. The hat symbolizes his mother who he loves so, he gets attached to it after a few years and even covers it with a shower cap.

What is under the hat?

: to keep (something) secret : to not tell anyone about (something) I'll tell you what happened, but you have to keep it under your hat.

What is under DDS hat?

In a DVD interview with Edd, he stated that whatever is under his hat "shines like a thousand suns": Hairpins; a large amount of these shiny metal pins is probably needed to maintain his hair.