For a long time I believed こもん to be コモン, from the English word ‘common’. Why? Because at my sports club this word was used for the person in charge and I assumed this was some kind of katakana version of the same English word i.e. common leader of a group.
To my surprise I’ve discovered that there is kanji for こもん, and therefore it’s actually a Japanese word:
こもん (顧問) a consultant; an adviser