Cockney term for someone without a fixed address. Usually used as a derogatory term for gypsies but some people (usually from Kent) say that there is a difference between a gypsy and an actual pikey - according to them, anybody can be a gypsy and wander around but a true gypsy is one who hails from the Balkans. However, the word "pikey" is used very liberally so the distinction isn't really viewed as being canon.
It's been said that the word "pikey" comes from the Spanish word "picaro" which means thief or an untrustworthy person.