aestas.

According to De Vaan, the term probably derives from Proto-Italic *aissāts, itself from Proto-Indo-European *h₂eydʰ-teh₂t-s, from the root *h₂eydʰ-. De Vaan proposes that the form *aissāts then underwent the analogical restoration of the suffix -tās during the transition to Latin. De Vaan criticizes an alternative etymology from *h₂e-h₂idʰ-eti, an i-reduplicated thematic present from the same root. This theory proposes that the aforementioned PIE form yielded *aīð-tu-, whence *aistu-, which is itself supposedly the source of both aestās and aestus. However, De Vaan argues that the existence of this reduplicated disyllabic PIE term is "unfounded." Moreover, De Vaan notes that -dʰt- becomes -ss- in Latin rather than -st-, preferring instead *h₂eydʰ-teh₂ts Proto-Italic *aissāt-s, which then had the suffix -t- consonant restored. Cognate with Latin aestus, Ancient Greek αἴθω (aíthō), and Old English ād (“pyre”).