1904 Chicago: What was then "anti-can rushing" ordinance?

I was doing some genealogy research last night and found a note about an ancestor from the Chicago Tribune in 1904.

"CHICAGO - The first arrest under the 'anti-can rushing' ordinance was made yesterday when Michael Keniry, saloonkeeper, was arrested on complaint of a probation officer that he sold a pail of beer to a 12 year old boy"

What was the "anti-can rushing" ordinance?