This compound noun terpi-kéraunos, interpreted here as ‘he whose bolt strikes’, is an epithet that applies exclusively to Zeus: a parallel epithet, also applied exclusively to Zeus, is the compound noun argi-kéraunos at Ι.19.121, I.20.016, I.22.178, which can be interpreted to mean ‘he whose bolt shines’ (GMP 195). Both epithets fit Zeus in his role as a thunder-god. The second part of both compounds terpi-kéraunos and argi-kéraunos is clearly derived from keraunós ‘thunder’, and the first part of argi-kéraunos is clearly related to arg-ós (earlier *argr-ós) ‘shining, speeding’, but the first part of terpi-kéraunos seems at first unclear. Related forms in other Indo-European languages, however, help elucidate the meaning of terpi- as combined with keraunós. In the case of keraunós, we find in the Baltic and Slavic branches of Indo-European the parallel forms perkūnas and perunŭ respectively, both of which are nouns meaning ‘thunder’ and/or ‘god of thunder’. Although the Greek and the Baltic/Slavic roots here, *kerh2(u̯)- and *per(kw)- respectively, are different in form, they are parallel in meaning, ‘strike’, and the morphology of their suffixation is also parallel (GMP 194–195). As for the terpi- of terpi-kéraunos, we find a comparable form in the Italic branch of Indo-European: it is the Latin noun quercus, meaning ‘oak tree’. This form, it can be argued, refers to the defining sacred moment when a thunderbolt strikes an oak tree: here the root of the u-stem noun quercus, from *perkwu-, is *perkw-, meaning ‘strike’ (GMP 186). So, terpi-kéraunos can be derived from *kwerpi-kéraunos via metathesis from *perkwi-kéraunos.