Tough, wild, mean. This expression dates from the mid-1800s, especially during the 1860s construction of the Union Pacific Railroad. As the track was extended into virtual wilderness, freight cars moving forward as track was laid transported a town. It consisted of tents occupied by the construction workers, liquor dealers, gamblers, prostitutes, and other camp followers of a rough and unsavory nature. Hence the town was called “hell on wheels.” Later the term was adapted for any individual or organization considered very aggressive, as in “On Monday mornings my boss is hell on wheels.” It also has been used admiringly, as in “Don’t underestimate her; she’s hell on wheels for getting things done.”