used by teenage boys to signal if he thinks a female is 'tight'. instead of screaming it out he can guess at how tight she is such as '5p' which means the girl is as tight as a 5 pence piece. if you come across a fat slag then she will probably be a £5 coin, about as big as the palm of your hand.
*tom spots girl*
tom: toby!! toby over here!!
*toby looks around and clocks the girl*
toby: holy shit 5p!!