Thick Mick is a slur used to describe Irish in a derogatory way. Commonly used in England where Irish immigrants did much of the manual labour. Thick meaning stupid, Mick as an abbreviation of Michael, a common Irish Christian name.
The word Thick Mick is a racial slur that has been used historically to degrade and dehumanize irish. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to irish. If you are unsure about whether or not it is appropriate to use the word Thick Mick, it is best to err on the side of caution and avoid using it in any situation.