heat of high summer; midsummer heat
Refers to the intense, often oppressive heat typical of the peak summer period.
We played outside, undaunted by the midsummer heat.
Without air conditioning, the heat of high summer is unbearable.
Compound of 真夏 (midsummer) + の (possessive particle) + 暑さ (heat, warmth). The exact historical derivation is straightforward; the phrase is a transparent combination of its parts.