Mid 18th century: from Scottish Gaelic feileadh-beag ‘little kilt’, from feileadh ‘plaid’ and beag ‘little’.
William wore his filibeg on Fridays unless it was a special occasion. This week, Wednesday was Wanda’s birthday. The plaid was perfect for the occasion.
Filibeg strikes me as a word we ought to have brought into English instead of the much simpler sounding “kilt”. Still, language grows by absorbing words with no planning. Some words slide in easily and seem to have always been there while others remain “foreign”.
One of my favorite absorbed words is “pajamas”, the word from Urdu and Persian by way of Hindustani and the time of the British Empire in India. It is so ingrained in English that it has many of its own derived offshoots: jammies, PJs, etc. It is so much a routine word, that it probably gets used every day by any family with children. “Go get your pajamas on. It’s time to wash up and get ready for bed.”