Archaic noun for the peak of summer. In modern Japanese, 真夏 (まなつ) is the standard word.
古い和歌に「夏中」と詠まれた語は、真夏の盛りを指す。
In old waka poems, the word 夏中 refers to the height of midsummer.
Compound of 夏 (summer) and 中 (middle). The reading なつなか follows older phonological patterns; the exact historical derivation is uncertain.