Tough, aggressive, wild, or mean, as in Watch out for the boss—he's hell on wheels this week. This expression originated with the building of the Union Pacific Railroad in the 1860s, when it denoted the last town on the line, which was carried on freight cars as the track was extended. The town consisted mainly of tents occupied by construction gangs, liquor dealers, gamblers, and other camp followers known for their rough and often vicious ways.