In many parts of the world the word rats is used as a mild swearword or expletive. There are several possible reasons for this. 'Drat' was first used early in the 19th century. It was a shortening of 'od rat' which was a euphemism meaning God rot. Drat has been adapted to rats. This definition comes from the new Oxford American dictionary.

Another reason that the name of the rat is used as an expletive is because rats have a strong negative association with disease and dirt. This may have started with 19th century urbanisation. Rats are still widely associated with dirt, sewers and the spread of the plague. This could certainly be one of the reasons that the word rats is used in a negative way.

In the Isle of Man off the north-west coast of England the word rat is taboo. People avoid using it as it is considered to be bad luck to mention the word. According to Wikipedia, the origin of this superstition is from maritime history. Sailors considered mentioning certain words to be bad luck. Instead, the word 'longtail' is used as a euphemism.

Dr Toby Bateson for Hammer Technologies Ltd