An abbreviation of “Start digging in yobutt, twin”, specifically coming from a YouTube video where robot SpongeBob said the phrase (insert some sort of potentially overused “HE SAID IT!” joke here)
Plankton: I command you to make me a Krabby Patty!
Robo-SpongeBob: poor imitation of clanker noises SDIYBT
Plankton: what? WHAT!?
Robo-Spongebob: it means poor imitation of clanker noises Start diggin in yobutt, twin
Plankton: oh ok, wait what!? WHAT!?