(young child) Hey mom. I'd like a pet for Christmas.
(parental guardian) Okay, sweetie, what do you want?
(young child) I was thinking a Donghae, is that okay?
(parental guardian) Sure.
Lee Donghae, or Donghae on stage, is a member of the 15 member Korean boy band Superstars - Super Junior.
He is one the group's visuals - along with Siwon - and is the partner of Lee Hyukjae, also known as EunHae (Eunhyuk and Donghae) OTP or One True Pair. He is also an actor. One of the main dancers of the group (Eunhyuk and Shindong). Kyuhyun once said during their Don't Don performance that he looks likes he's going to destroy the stage.
He is also member of SJ's sub-group SJ M and Super Junior Donghae and Eunhyuk.
Donghae is a name for the "East Sea" or "Sea of Japan." It's a body of water that essentially connects the two countries (that historically don't get along).
Hence: Donghae (when used in weeb slang) means "After a slow burn (a long waiting period) your OTP finally get together." It's meant to be used as more of a sound effect or to punctuate.
Weeb 1: IT TOOK ED 64 EPISODES TO GET WITH WINRY! FUCK! WE'RE FINALLY HERE!
Weeb 2: DONGHAE!!!