i-adjective
extremely hot; damn hot
Colloquial and emphatic; クソ acts as an intensifier, similar to 'damn' or 'freaking' in English. Used for weather, objects, or situations that feel unbearably hot.
See also: クソ
今日はクソ暑いから、エアコンをつけっぱなしだ。
It's so damn hot today, I've got the AC on nonstop.
この部屋、クソ暑くない？窓開けよう。
Isn't this room freaking hot? Let's open a window.
Compound of the intensifying prefix クソ (from 糞, 'excrement', used as a vulgar intensifier) and 暑い ('hot'). The prefix クソ is productively attached to adjectives for strong emphasis.