A small class of English words derived from the Latin suffixes -arius/-aria/-arium, meaning “connected with” or “pertaining to,” can be identified by the French descendant -aire. Here is a summary of those terms as used in English.
The primarily British English term commissionaire refers to someone who performs commissions. (That word is used in the sense of “tasks.”) Debonair was originally the French phrase de bon aire, meaning “of good family, nature, or race”; it was applied originally to well-trained hawks, later pertained to people of a courteous nature, and was then revived (after that sense became obsolete) to refer to a confident, sophisticated man.
Doctrinaire describes a dogmatic, headstrong person (the root word is descended from the Latin word doctor in its original sense of “teacher”). Extraordinaire is an adjective that, in deference to its French origins, is often located after the noun it modifies; its root word, meaning “regular” or “usual,” ultimately stems from ordo, the Latin word from which order is derived.
Millionaire is based on million, from the Italian term millione, meaning “a great thousand” (a thousand thousands); it refers to someone whose wealth amounts to at least a million dollars. By extension, a billionaire is someone who has a billion dollars or more; inevitably, there will eventually be trillionaires.
Legionnaire derives from the Latin legion, which stems from legere, a verb meaning “gather” or “select.” A legion was the basic military unit in ancient Rome, and the French adopted the term and formed legionnaire to refer to a soldier. In English it is associated with the personnel of France’s Foreign Legion, and in the United States it is known as part of the name of Legionnaire’s disease, so named because the first outbreak occurred at a convention of the patriotic organization known as the American Legion.
Solitaire, from the Latin word solitarius, meaning “alone” or “isolated,” came to refer to a recluse or a widow and then later a single gem, but now it is mostly associated with a card game one plays by oneself.
The French word affaire, adopted into English in the diplomatic title “charge d’affaires” (which refers to a deputy ambassador or minister) is unrelated, as is the English form affair; they are descended from the French phrase à faire, meaning “to do,” and are related to facile and fact. Luminaire, from the French word for “lamp” or “lighting” and referring to a lighting unit, also has no connection; it’s from the Latin word lumen, meaning “light.”