To quote someone who knows what they're talking about:
"From Arabic harIm (literally "something forbidden") and
haram (literally "a sanctuary")"
Although its original meaning was the part of a household reserved for females - including perhaps a man's mother, sisters, and daughters, as well as wife or wives - it is commonly used to mean a group of more than one woman who are all available sexually to a single man. This situation is
Polygyny ("multiple women.") Sometimes it also refers to a group of men who are available to a single woman. This situation is
Polyandry ("multiple men.") Either may be called
Polygamy ("multiple marriage.")
People who think it derives from Latin don't know what they're talking about, which is no great crime - no one knows everything. People who think it derives from Latin and say so on a
public forum like this not only don't know that they don't know what they're talking about, but are displaying and perpetuating their ignorance.