When English speakers say that someone is “all heart,” they are using this basic idiom to say that the person is full of positive attributes like love, charity, goodwill, and generosity. Those in English speaking societies often use this phrase and other similar ones to complement an individual. For example, someone might say that someone else “has a lot of heart” or “is big-hearted.” A slightly different use of the word is common in expressing total commitment or fondness, where “with all my heart” is a common phrase.
The use of the word “heart” to refer to human emotions is ancient and common to many languages around the world. For centuries, the idea of a visual heart depicting love has been a mainstay of personal expression. On Valentine’s Day, a long-standing tradition in English speaking societies, this idea is most prominent. A simple drawing of a symmetrical shape with two rounded tops has been used to represent this “emotional heart” and shows up everywhere from a Valentine’s day gift to a deck of playing cards.
Another use of the word “heart” in English is related not to love, but to another different set of human emotions. Speakers of modern English might say that someone “has heart” or “has got a lot of heart.” This generally refers to someone with courage, fortitude, or determination. In many cases, speakers use this phrase, which is slightly different than saying "all heart," to refer to a person’s bravery, with positive and negative forms applying. For example, saying someone “doesn’t have the heart” to do something often implies that the person lacks the strength or will to do something. A similar phrase, “[his or her] heart wasn’t in it,” implies that the person took an action without believing in it.
In general, being "all heart" or having “heart” is a positive expression of desirable values in a person. As mentioned above, this can easily be changed to the conditional form, i.e. “[he or she] has a big heart.” In some rare cases, English speakers might also refer to someone being “lion-hearted” or “having the heart of a lion.” "All heart" and similar phrases work on the same basic concept: that the heart, aside from being a physical organ that pumps blood, is somehow responsible for the gamut of human characteristics that make up a person’s individual personality.