Initials of "Disregard that, I suck cocks", a phrase from 4chan
used on the internet when someone wants to take back what he said, e.g. when someone finds out something that contradicts what they previously posted.
Because the initials don't have the swearing explicitly written, this form can be considered more polite than the full phrase.
Alice: I think Bob told this one joke, I'm almost sure about that...
*she finds out Charlie told it, not Bob*
Alice: DTISC, it was Charlie.