Such a small, but such an important word: yes.
Perhaps because of the need to have a strong word to indicate consent or affirmation, this word has been with us a long time. We find it in Old English as gise or gese, which was used as “yes”, but literally meant “so be it”. Interestingly, both forms can be used together, such as in gise, lá gese (yes, oh yes!), from King Alfred’s version of Boethius.
It was formed from gé or géa, meaning “so, thus”, with the simple addition of si, from the verb beón, which means “to be”.