Old Irish fius

← Proto-Celtic *wissus (inherited)

← Proto-Indo-European *weyd- (root)

← Proto-Indo-European *wéydtus (inherited)

← Proto-Indo-European *weyd- (mention)

→ Welsh Arthwys (mention)

→ Irish fios (mention)

→ Scottish Gaelic fios (mention)

→ Manx fys (mention)

→ Middle Irish fis (inherited)