 In the fifth century BC, Herodotus referred to "Keltoi" living around the head of the Danube and also in the far west of Europe.The etymology of the term "Keltoi" is unclear.Possible roots include Indo-European *"kʲel" 'to hide' (present also in Old Irish "ceilid"), IE *"kʲel" 'to heat' or *"kel" 'to impel'.