The true reason why an amateur radio operator is called a 'ham' is not known - the truth has been lost to the past ages.
Various speculations do exist, however - each with a Some degree of believability.
Here are the most common. This was in a time when any legal administration, red tape or actual assigned callsigns were in their infancy. In addition, the government was attempting to gain control and administration of the entire radio wave spectrum. Early amateurs had quite a fight to maintain any radio spectrum for their experiments. Supposedly, an impassioned speech was made on the floor of the US Congress in behalf and support of amateur radio operators and their stations.