The Americans and Canadians referred to Germans, especially German soldiers as "Heinies", from the pet form of the common German male proper
name Heinrich.
In the film 1941 the Slim Pickens character calls a German Officer "Mr. Hynee Kraut!"
Heini is actually a common German slang word with a slight derogatory meaning similar to
moron or
idiot, but it could be of different origin.