Tonk is a slur used to describe Mexicans in a derogatory way. Illegal Mexican immigrants. "Tonk" is the sound that is made when the illegals are hit over the head with the large flashlights/batons that the US border patrolmen carry.
The word Tonk is a racial slur that has been used historically to degrade and dehumanize mexicans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to mexicans. If you are unsure about whether or not it is appropriate to use the word Tonk, it is best to err on the side of caution and avoid using it in any situation.