Note: Slugging
http://en.wikipedia.org/wiki/Slugging
("Origin of the term[:] The term slug (used as both a noun and a verb) came
from bus drivers who had to determine if there were genuine passengers at
their stop or just people wanting a free lift, in the same way that they
look out for fake coins—or 'slugs'—being thrown into the fare-collection
box.")
I am glad Wikipedia explains it, for I can not find this definition in the
web dictionaries that I try.