It’s the Yiddish translation of the passover holiday. It is the same reason for many other commonly used phrases in the Orthodox world that have no source in classical Judaism or are simply grammatical errors
Shaloch manos (as opposed to Meshloach manos)
Shabbosim
Zechusim
chanukah mesiba (as opposed to mesibas chanuka)
chal hamoed
shlashudos
I am sure the oilam can add to the list…