Why the abbreviation "maitre d'"?

foreign-phrases

It appears that it is simple an abbreviation which was used from 1942 probably when he term was used mainly to refer to the manager of a dining room:

By the late 19th century, the term was being used to refer to a hotel manager; in the 20th century, it came to mean the manager of a hotel dining room and eventually a headwaiter.

Maitre d'hotel:

1530s, "head domestic," from French maître d'hôtel, literally "house-master," from Old French maistre "master; skilled worker, educator" (12c.). Sense of "hotel manager, manager of a dining room" is from 1890. - Shortened form maître d' is attested from 1942;

Plural of maitre d':

When a compound word is split into parts, with or without hyphens (like mother-in-law or attorney general), the plural ending traditionally goes on the most important part (mothers-in-law or attorneys general).

But maitre d’ is a special case. The plural of the full version is maîtres d’hôtel, as one would expect, but the plural of the shortened form is maitre d’s, according to The American Heritage Dictionary of the English Language.

Note that: