Cowgill suggests it might be a semantically degraded form of *esti "is", while Schrijver (1994) has argued it is derived from the particle *eti "and then", which is attested in Gaulish.
Cowgill suxire que talvez sexa unha forma semanticamente reducida de *esti 'é', mentres que Schrijver (1994) argumenta que *(e)s é derivada da partícula eti 'e entón, que está testemuñada no galo.