Some automotive historians say that the term originated with stolen vehicles being refitted with another engine and repainted. In the early days of automobile manufacturing there was no identical matching transmission, body frame, and engine numbers. It was possible to change engines and repaint the car or truck and in effect turn it into a different vehicle and thus it became near impossible to prove that the vehicle was stolen. The term “hot” was equivalent to being stolen. The term “rod” was equivalent to any motorized vehicle. Even today, the Ontario Ministry of the Environment in its vehicle emissions regulations, refers to a “hotrod” as any motorized vehicle that has a replacement engine differing from the factory original. Another possible origin includes replacement of the camshaft with a new (“hotter”) version, sometimes known as a hot stick or hot rod. Roadsters were the cars of choice because they were light, easy to modify, and inexpensive. The term became commonplace in the 1930s or 1940s as the name of a car that had been “hopped up” by modifying the engine for higher performance. A term common in the early days was “gow job”. This has fallen into disuse except with historians.
The gow job morphed into the hot rod in the early to middle 1950s.
source : wikipedia