Peanuts by Charles Schulz for January 25, 1960
Transcript:
Charlie Brown and Linus walk to school. Linus says, "Rats! I forgot the egg shells!"(BR) (BR) Linus says, "Miss Othmar wanted us to bring some egg shells to school today . . . We were going to make iglii . . ."(BR) (BR) ; Charlie Brown asks, "Igli?" Linus answers, "That's plural, Charlie Brown . . ."(BR) (BR) Linus says, "One igloo . . . two igli!"(BR) (BR)
I can’t think of any linguistic construction that would produce that plural form. Most irregular plurals that take the -i suffix end in -us and come from Latin roots, like radius → radii, alumnus → alumni, fungus → fungi. Igloo is neither a Latin root nor does it even end in -us, so it should just be a regular plural by any rule.