To blow hot and cold is an idiomatic English expression that generally refers to a person who tends to be changeable in his opinions or mood. Phrases such this generally take on a different meaning than the definition of the words themselves. For example, the words hot and cold generally will be a reference to temperature. To say a person blows hot and cold, however, refers to his state of mind, his thoughts, or his temperament.
Many phrases like this are centuries old. Some have roots in stories or superstitions passed down from generation to generation. The phrase to blow hot and cold probably does not refer to a superstition or fear, but it is a very old expression that might have originated around 500 B.C..
When someone uses the expression to blow hot and cold in reference to another person, he is generally describing someone who frequently changes his mind. For example, a person who blows hot and cold may be indecisive about what career he'd like to pursue. He might also constantly change his mind about his preferences and favorites.
An individual who blows hot and cold might also be prone to constant changes in mood. He might be exuberant and animated, then turn sullen and withdrawn the next day. He might show contentment and pleasure one day, then resentment or anger the next.
Blowing hot and cold can describe a person whose opinions on various matters constantly change. As an example, the person who fits this description may claim he's against the institution of public zoos. He might change his mind on this topic, however, and claim he believes holding wild animals in a zoo environment is a positive thing he supports completely. The following week he might say zoos have a negative impact and he doesn't believe they should exist. By faltering in his opinions, he could be perceived as blowing hot and cold.
Inconsistencies in a person's behavior and emotions can also be perceived as someone who blows hot and cold. For instance, a man who pledges his love and devotion to a woman, then a few days later claims he is fearful of commitment and becomes emotionally detached, might be described as someone who blows hot and cold. This person may be loving and committed one day, then distant and cold the next. He might be described as someone who is likely to blow hot and cold because of his changing emotional state.