Ami is a slur used to describe Americans in a derogatory way. This is just a German abbreviation. By itself it is not derogatory. Scheiss-Ami = Shit American, obviously derogatory.
The word Ami is a racial slur that has been used historically to degrade and dehumanize americans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to americans. If you are unsure about whether or not it is appropriate to use the word Ami, it is best to err on the side of caution and avoid using it in any situation.