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.