When you launch semen in a girl's booty and throw her on the sand. The sand or dirt* then sticks to the released semen and creates a sandy ring or "sandy bagel" around the butthole.
Talking for an extended period of time on one subject, so much so that it begins to lose its meaning and take on a completely different subject, or sub-subject.
person one "we sand bagel so hard"
person two "I laugh when the bagel gets sandy"
person three "sand bagel."