The Avestan word xšuuaš
Word xšuuaš
Meaning six, 6
See also Proto-Indo-European: *kswek̂s ‘six’

Other words that might be of interest
aēuua ‘one’, ašta ‘eight, 8’, θritiia- ‘third’, paoiriia- ‘first’, paṇca ‘five, 5’, bitiia- ‘second’, hapta ‘seven, 7’, caθβarasca ‘four’