The Proto-Celtic word *swexs
Word *swexs
Meaning 6, six

Other words that might be of interest
*oino ‘one’, *dwei ‘two’, *dwā(w) ‘two’, *nawan ‘nine, 9’, *tri ‘three, 3’, *kʷetru ‘four’, *kʷenkʷe ‘five, 5’, *sextam ‘seven’