From Proto-Indo-European *gʷr̥Ht-, *gʷerH- (“to welcome, greet, praise”). Cognates include Sanskrit गृणाति (gṛṇā́ti, “to praise”), Old Church Slavonic жрьти (žrĭti) and Old Prussian girtwei (“to praise”).
This noun originally appeared only in the nominative and accusative plural (The genitive, dative, and vocative plural are unattested and ablative plural only rarely) and was used with agō when rendering thanks to the gods. grātiās agō was generally used for thanks between humans.
Third declension, i-stem.