About 97 degrees Fahrenheit. Unlike, say, "hot as hell" or "scorching
hot," this expression is actually fairly precise, since the temperature of
one's balls is carefully regulated by the
scrotum to maintain a high sperm count.
Alice: Man, it is
hot as balls out there today.
Bob: Actually, it's about 99 degrees, so it might be "
hot as lungs," but balls are slightly cooler.
Alice: ...ooookay, whatever.