petrolhead /petrlhed/ noun (BrE, informal) a person who is extremely enthusiastic about cars and driving, especially fast cars and motor racing, and who does not want to use any other sort of transport: Will petrolheads ever leave their cars? We are a petrolhead society. Speed may thrill petrolheads, but many pressure groups are keen to point out that it kills too. This new car will set any petrolhead’s heart pumping, and there is little doubt that it will sell. To drive your choice of top motor would be petrolhead heaven.
Petrolhead is an example of a compound noun, which is a noun that is made by combining two other words (in this case the nouns petrol + head). Like the words crackhead, cokehead and soccer-head, it describes addiction to or great enthusiasm for something. Another similar word is Motörhead, the name of a famous heavy metal band.