A single black dot tattooed on the back of the hand (often between the thumb and forefinger), or multiple dots - sometimes one on each finger.
The dots were traditionally made with a school compass or other sharp implement and ink poured into the hole. Tattoo artists are an option for those talented enough to have managed to steal a few quid from a granny.
They are worn by particularly brain dead and tragic denizens of Portsmouth (aka Pompey) - a depressing town on the South coast of England.
Owners of Pompey dots are very likely in-bred, doubtless mentally deficient, and definitely chavs. They should be considered dangerous, and certainly chlamydia carriers. Avoid them at all costs.
"Check aaht ma pompey dot ma! Innit fackin pukka! Aahm portsmuff born an bred, me!
Y'up fa Route 66 t'night?*"
*The age gap between generations of Portsmouth families is usually around 13 years. Accordingly, 3 generations of Portsmouth slappers are regularly seen dancing together around piles of handbags in local night clubs.