charge d'affaires (n.)

early 13c., "to load, put a burden on or in; fill with something to be retained," from Old French chargier "to load, burden, weigh down," from Late Latin carricare "to load a wagon or cart," from Latin carrus "two-wheeled wagon" (see car).

Senses of "entrust," "command," and "accuse" all emerged in Middle English and were found in Old French. Sense of "rush in to attack, bear down upon" is from 1560s, perhaps through earlier meaning "load a weapon" (1540s). Meaning "impose a burden of expense" is from mid-14c. That of "to fix or ask as a price" is from 1787; meaning "hold liable for payment, enter a debt against" is by 1889. Meaning "fill with electricity" is from 1748. Related: Charged; charging.

c. 1300, "what one has to do, ordinary business," from Anglo-French afere, Old French afaire "business, event; rank, estate" (12c., Modern French affaire), from the infinitive phrase à faire "to do," from Latin ad "to" (see ad-) + facere "to do, make" (from PIE root *dhe- "to set, put").

According to OED a Northern word originally, brought into general use and given a French spelling by Caxton (15c.). General sense of "vague proceedings" (in romance, war, etc.) first attested 1702. Meaning "an affair of the heart; a passionate episode" is from French affaire de coeur (itself attested as a French phrase in English from 1809); to have an affair with someone in this sense is by 1726, earlier have an affair of love:

'Tis manifeſtly contrary to the Law of Nature, that one Woman ſhould cohabit or have an Affair of Love with more than one Man at the ſame time. ["Pufendorf's Law of Nature and Nations," transl. J. Spavan, London, 1716]

Related: Affairs.