Inherited from Middle French glaise, glase, from Old French glaise, gloise, of obscure and uncertain origin. According to one theory, derived from Gaulish *glisa, which is attested in Late Latin glissomarga, glīsomarga (“a kind of marl, white marl”), although the precise meaning of *gliso- is uncertain.
Alternatively, perhaps derived from a Germanic language, compare Dutch klei, German Low German Klei, English clay, all from Proto-West Germanic *klaij.
Or, possibly from Latin glis (“thick clay, tenacious earth”).