'Summer' is one of two ancient season words inherited directly from Proto-Indo-European.
The warmest season of the year, between spring and autumn.
From Old English 'sumor' (the warm season, the period from June to August), from Proto-Germanic *sumaraz (summer), from PIE *sm̥h₂-ró- or the base *sem- (summer, the warm or productive half of the year, possibly one or together as the prime unified season). The Sanskrit cognate 'samā' means season or half-year, and Avestan 'hama' means summer, both preserving the ancient sense of summer as a temporal unit. Old Irish 'sam' (summer), Welsh 'haf' (summer), and Armenian 'amarn' (summer) all reflect the same