Its source is leas cuillinn, a Gaelic name meaning "Holly garden."
This was the surname of a Scottish noble family closely associated with the royal house of Stewart.
It was popularized as a first name by Hungarian-born English actor Leslie Howard (1890-1943). As Ashley Wilkes in the 1939 film version of Gone With the Wind, Howard succeeded in popularizing both his personal name and his character's name.
In recent years both Ashley and Leslie, though still used for boys, have been more commonly given to girls.