Tuna is a slur used to describe Portuguese in a derogatory way. Portuguese in San Diego worked principally as tuna fishermen.
The word Tuna is a racial slur that has been used historically to degrade and dehumanize portuguese. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to portuguese. If you are unsure about whether or not it is appropriate to use the word Tuna, it is best to err on the side of caution and avoid using it in any situation.