its not the heat its the humidity

its not the heat its the humidity is a phrase used when someone wants to explain that what they think has gone wrong is actually due to another thing which has caused the trouble. This idiom refers to the fact that when it is humid and the air is damp, the temperature can feel even hotter and more miserable than just heat itself. Figuratively, this means that one bad thing is making the situation feel worse than why you think it is that way.