It is a very old expression. Hot water was used five hundred years ago to mean being in trouble.

VOA: special.2009.05.10

Theory was both hot and cool, and it was something about which, following from that, one had not just opinions but very, very strong opinions.

理论很热门，也很酷，对于理论，人们不光有想法，而且是非常强烈的想法。

耶鲁公开课 - 文学理论导论课程节选

It was about a relationship between a recent college graduate and what popular culture today would call a "hot mom" -- a sexy older woman.

VOA: special.2009.05.04

Each hot cake was so large that it took five men to eat one.

VOA: special.2009.01.24

It was a hot day, and the road was dusty.

VOA: special.2009.08.27