'Amateur originally meant athletes that competed unpaid, simply for the enjoyment and competition. This was opposed to Professionals who were paid to play.
The Canadian Amateur Hockey Association, in 1937, defined amateur as a player who might get paid for his play, but had to depend on an outside job for a living. In other words, he was not paid a lot for hockey. The IIHF eventually accepted this definition in the 1950's.
The Olympic Games had a strict definition of amateur - no pay at all - until it opened up in the 1980's.