To have a heart of glass means to be very easily affected by something or someone. Often, this idiom is used to describe a romantic heart that is easily hurt or broken. A person with a heart of glass is generally regarded by others as being extremely sensitive and emotionally fragile.
English sayings like this have been used for generations. Such idiomatic expressions are commonly understood and, thus, a part of everyday American English. They are so common, in fact, that seldom are their meanings or origins ever even questioned. While some who are not used to using or hearing idioms like these may be led to believe one is referring to a literal heart made out of glass, such is not the case at all.
It is not unusual for common sayings like these to be used in popular music and film offerings. One such example is the song titled Heart of Glass made popular by the American music group Blondie. The song, which topped US and UK music charts in the late 1970s, tells the story of a person who thought true love had been found, only to be very disappointed in the end.
Heart of Glass was also the title of a 1976 film directed by Werner Herzog. In this instance, however, the meaning of this phrase may not be quite the same as the American usage. Instead, it refers to the plot about secret glassblowing methods that were lost when a foreman at a glass blowing plant died. The film’s title, however, may be a double entendre, as the foreman lived in a town that is renowned for producing a special type of glass, known as "ruby glass." As a result of the foreman’s death, the town’s economy suffered greatly.
A person who has a heart of glass is usually easily hurt by others. This person is known to be very emotionally fragile and often needs special handling in order to avoid offending. A similar expression that may apply to a person with a heart of glass includes a person who wears her heart on her sleeve.
It’s not immediately known where the idiom originated. The similarities between human emotion and glass, however, are not hard to imagine. For instance, items made of glass are typically fragile and require special handling. In this same way, the human heart often exists, particularly in situations of romantic love. Both may be easily broken if not handled with special care.