Originally Posted by gum

against that passengers sitting in the old two class times (First and Coach) they created that arrogant and unfriendly abbreviation in German
M = MINDERZAHLER (a person paying less than the full fare) to tell everyone that these are not good and full paying passengers but passengers which are not worth the full service they get in the coach class which was a mixture of all fares 100%business, 75% weekend, 50% flieg und spar and so on (all sitting in the same class(

So due to that historic calling Lufthansa until nowadays is the only airline using M instead of Y.