Why is the plural of “hibou” spelled “hiboux”?

French nouns ending in -ou normally just add -s: un trou, des trous. Seven of them take -x instead, and French schoolchildren memorise them as a list: bijou, caillou, chou, genou, hibou, joujou, pou — jewel, pebble, cabbage, knee, owl, toy, louse. So one owl is un hibou and two are des hiboux, with an x you cannot hear and have to remember.

The x is usually traced to medieval scribes, who wrote the -us plural ending with an abbreviation that later got read as the letter x — the same accident that left one on genoux. Nothing links the seven words in meaning; they are simply the ones that kept the old spelling. Every other -ou noun takes a plain -s: clous, fous, sous, verrous, kangourous.