While having sex on the beach quickly stick your penis in the sand then re-insert it into your partner and watch them flap around like a seagull.

A critical manager or supervisor who makes brief appearances to put on a meaningless but unpleasant display for employees to try to show they're doing their job. They swoop in, make a lot of noise, shit on everything, then fly away.

Any person that eats free leftover scraps of food no matter how disgusting it is.

Masturbating in your hand and then throwing it in someones face!!!

When you whack off and cum in your hand, then you run up to someone, slap them in the face with your salty hand and shout "Seagulled!"

Past tense for a victim of seagulling wherin the person has had cum thrown at them by a random person or stranger for a laugh.
This person may have had their experience filmed to make the situation more degrading/hilarious depending on perspective

a relatively new fad among british school boys, whereby they masturbate furiously during recess, collect the jizz in their palms, find a younger unsuspecting boy and slap his face while shouting "SEAGULL!"