This from WIikipedia:
The noun Polack (/ˈpoʊlɑːk/ or /-læk/; also Pollack, Pollock, Polock), in the contemporary English language, is a derogatory reference to a person of Polish descent. It is an Anglicisation of the Polish language word Polak, which can mean a Polish male person or a person of Polish nationality (feminine being Polka), with a neutral connotation. However, the English loanword "Polack" (note the spelling difference which does not appear in Polish) is considered an ethnic slur in the United States and the United Kingdom, and therefore is considered insulting in nearly all modern usages.
Slang: Disparaging and Offensive (Random House Unabridged Dictionary)
Offensive Slang used as a disparaging term for a person of Polish birth or descent (The American Heritage Dictionary).
Germans also use the derisive term Pollacken (Pollacks) and go one step furtehr by creating a corresponding country: die Polackei (Pollackland).
Was it German immigrants to America who preceded the Poles that first introduced the term Pollack? Das weiß ich nicht.
(I know not.)