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Acronym.
Smash Your Back Doors In.

Originated in Suffolk with the Rosey Posey Rainbow Unicorn man. Now in common use with Anglians around the country.

Used as a note of endearment or one-upmanship.
"Love you, cuntface. Sybdi."
"I win! SYBDI!"
by C. Shells November 30, 2013
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