Humdinger can refer to:

• An old boxing term, meaning a person would hum when dinged in the head. Allii was known as the king humdinger, and his motto was fly like a butterfly sting like a humdinger; it was changed to float like a butterfly sting like a bee when a group of activists were offended by the violence in the term "humdinger".

• An electrical circuit designed to reduce mains hum. Subsequently, anything of outstanding value or performance.

In the early 1900s, radios used electronic valves (tubes) in which the cathodes were heated by direct current (dc). When mains electricity became available, the ac used to heat the cathode produced a very loud hum (far louder than the required signal). The circuit which reduced this hum was called a humdinger.

Later, the indirectly heated valve was designed. This separated the cathode from heater and the hum problem virtually disappeared.

(The dinger on which the word is based was a tool designed to knock out dents or dings in car bodywork.)