Czischki
This combination of letters indicates rather a Polish name spelling than Prussian - Polish spelling for it is 'ciszki' and multiple variations, quite common sequence in Polish words. Much of towns names here probably contains such letters - couple of examples:
Cieszki, Cieszków, Cieszkowice, Ciszkowo, Cieszkowo, Ciszkowice, Czyżkówko, Czyszki, Czyszkówek, Czyżkówko, Franciszków and so on...