Coster is usually defined as short for costermonger: a street vendor of fruit, fish, vegetables, and so forth. The examples of usage shown in the OED (229) strongly imply that costermongers enjoyed a reputation for uncouth, bullying behavior. Knight (177) calls attention to the line from Shakespeare’s Henry VI : “Virtue is of so little regard in these costermonger times.” Samuel Johnson, in his 1756 dictionary (165), defines coster-monger as a seller of apples. Bosdêt (43) says they originally sold only French apples, which were far juicier than English apples and enjoyed a much longer shelf life. But, I wander.