“By heart” is an English idiom used whenever someone knows something completely without any gaps in his or her knowledge on the subject. This is an English idiom used whenever someone has such intimate knowledge of something that he or she can answer any question on the subject. The idiomatic expression “by heart” is also used when someone is able to recite something, be it a song or a piece of writing, without missing a single word. It is one of many idioms that make use of the heart as the figurative place where everything important to a person is stored.
On certain occasions, a person might use an English word or phrase which means something quite different from what someone might expect, based on the literal definitions of the words. Such a construction is known as an idiom, which actually takes its meaning not from any predetermined definitions but rather from the way the word or phrase has been used in a culture. It is, therefore, understood in this manner, and speakers can use it to add color and impact to their conversations with others. One idiom which makes use of the figurative associations of the human heart is the phrase “by heart.”
When someone knows something so well that it warrants the use of this phrase, he or she can be considered an expert on the subject. Although there is often some exaggeration at play when this phrase is used, the general implication is that anyone who knows something in this manner knows it without fail. As an example, consider the sentence, “I’ve been living in this area for so long now that I know it by heart.”
In conjunction with this notion, the phrase is often used when someone is asked to recite or repeat some piece of writing or anything else that can be memorized. After working on learning something for so long, the words contained in it often can be recited without any struggle. For example, someone might say, “The teacher said we would have to learn all of those passages by heart if we were going to pass.”
The heart is involved in many idioms. Since the heart is the figurative center of emotions and all things that are important to people, any phrase containing it has a certain impact on listeners. “By heart” indicates a complete knowledge of a subject, so complete that it resides by a person’s heart.