noun
heat of high summer; midsummer heat
Refers to the intense, often oppressive heat typical of the peak summer period.
真夏の暑さに負けずに、外で遊んだ。
We played outside, undaunted by the midsummer heat.
エアコンがないと、真夏の暑さは耐えられない。
Without air conditioning, the heat of high summer is unbearable.
酷暑 means 'severe heat' and is often used in weather reports; 真夏の暑さ is a more everyday expression for the same concept.
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.