Waterproof Toaster

Listen up, buttercup. This ain't rocket science. So, the joke is that toasters ain't supposed to be waterproof. Tossin' a toaster in water is like signin' its death warrant, and probably yours too because it is plugged into an electrical outlet. The joke implies that the toaster owner found out the toaster was waterproof because it didn't break when it got wet, which is a morbid way of suggesting getting electrocuted.