Which common English word has no letters in its plural form in common with its singular?
I haven't expressed myself very well but I hope that I have made myself understood. If not then as an example COW and CATTLE could not be the answer as C is common to both words.
I have found the following statement that may or may not be relevant to your question . .
KINE (an archaic plural of COW) qualifies as a plural that shares no letters in common with its singular. Others are I/WE, ME/US.
Only a silly quibble, I know, but 'kye' - used in Scotland to this day to mean 'cattle'...ie effectively the plural 'cows'... originally meant singular 'cow'. I presume it was from thence that 'kine' developed rather than from 'cow'.
(I'm not trying to take anything away from your response, Sarumite! Honest!)