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A principle that is used to determine if a side bitch or FWB situation is getting too serious:

-Sleeping over
-Asking to be exclusive
-Kiss goodnight

If all four criteria are met at different points in the fling, you may have a relationship on your hands.
Are you sure you and her are just FWBs? Didn't you stay over at her place last night? You better watch out, that's one of the SACK Principles.

Yo dude you've met all four points of the SACK Principle! She totally wants you to ask her out.
by Sir sacksalot April 10, 2014
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