Abarim Publications' online Biblical Hebrew Dictionary
It's officially not known from which verb or action the important masculine nouns לבב (lebab) and לב (leb), both meaning heart, come from. Still, from the usages of the nouns we may readily deduce that it had to do with centralization, or the formation of a focus of any kind.
The Hebrew heart
To the Hebrews, a creature with a heart was a creature who was able to assess the kaleidoscope of impulses around him, sieve out the things that were most important and more or less disregard the rest. Heart-forming lies at the base of both intelligence and determination, and the opposite of having a heart is being either ignorant, indifferent or cowardly.
The form לבב is used very infrequently as denominative verb, meaning to make to catch attention, both emotionally (Song of Solomon 4:9) and intelligently (Job 11:12).
Since ancient times, this mind-central heart and the blood-pump that creatures usually carry in their chest have been associated with each other, although it's by no means certain that the ancients also believed that the mind sat in the physical blood pump. The Hebrews, after all, associated the mental heart equally with the penis (see below). But we can be sure that the ancients realized that one's heartbeat can be felt throughout the body, and from there it's not a huge step to realize that the heartbeat has something to do with blood (דם, dam), in which was the life of a creature (Genesis 9:4, Leviticus 17:10-14).
It's not clear whether the Bible writers were aware of an intrinsic difference between the nouns לבב (lebab) and לב (leb), but what is clear is that we aren't. They are used pretty much equally.
On very rare occasions they may denote the "heart" of something that's not human. In Jonah 2:3, Jonah laments that YHWH had hurled him into the "heart" of the sea, which not simply denotes the middle of the ocean, but rather the emotional heart of it (emotional from the perspective of Jonah); the place which is farthest removed from the "heart" of heaven (Deuteronomy 4:11). Besides the heart of heaven and the sea (Exodus 15:8, Psalm 46:2, Proverbs 23:34, 30:19, Ezekiel 27:4, 27:25-27, 28:2-8), Scriptures mention the heart of an oak (2 Samuel 18:14) and the heart of the enemies of the king (Psalm 45:5). All other of the more than 800 occurrences of לבב (lebab) and לב (leb) refer to qualities of humans.
The nouns לבב (lebab) and לב (leb) designate the inner person and is associated with one's flesh (שאר, she'er; Psalm 73:26) but mostly one's soul (נפש, nepesh; Deuteronomy 4:29, Joshua 22:5, Jeremiah 32:41). It's often endowed with qualities that we moderns ascribe to the mind (Genesis 31:26, Job 12:3, Ecclesiastes 9:3), such as thinking (Isaiah 10:7), memorizing (1 Samuel 21:13) and forgetting (Deuteronomy 4:9). The heart is the seat of determination (1 Samuel 7:3), courage (Daniel 11:25, Psalm 31:24, Amos 2:16), secrets (Psalm 44:21), conscience (Job 27:6) and morality (Deuteronomy 9:5), and all kinds of specified feelings (Leviticus 26:36, 1 Kings 8:38, Isaiah 30:29), but also of wickedness (1 Samuel 17:28), erring (Psalm 95:10), pride (Psalm 101:5) and arrogance (Deuteronomy 8:14).
According to Jeremiah, the heart is more deceitful than anything else (Jeremiah 17:9), yet the heart wakes when the rest of a person is asleep (Song of Solomon 5:2). It may be broken (Psalm 34:18), molten (2 Samuel 17:10), clean (Psalm 51:10), new (Ezekiel 18:31), fat (Isaiah 6:10), even double (Psalm 12:3).
The heart may be circumcised (Romans 2:29, Deuteronomy 30:6, Jeremiah 9:25), although it's never been properly explained what that means in practice, why a surgical procedure done to the penis could also be done to the heart, or why it was supposed to be done to the penis in the first place. From Deuteronomy 30:6 we learn that YHWH will circumcise our heart and the heart of our descendants, so that we will love him with all our heart and with all our soul, and so that we may live. The verb to circumcise is מול (mul II) and the word for foreskin is ערלה ('orla).
Besides the nouns לבב (lebab) and לב (leb), the following words are drawn from this same root:
- The (probably) feminine noun לבה (libba), probably also meaning heart. The rule is that body parts of which there is only one are masculine, while the pairs are feminine, which makes a feminine form of the word לב (leb) rather unexpected. It only occurs in Ezekiel 16:30, where YHWH makes an observation of Israel's harlot heart. The feminine version might indicate that God thinks that Israel has not one unified heart but a cluster of heart fragments.
- The noun לבבה (lebiba), which denotes a kind of dough-based products (2 Samuel 13:6-10). Most commentators seem to believe that these things were called such because of their shape, but it's difficult to imagine how a heart-shaped bun would look long before the familiar heart-shape was connected to the heart phenomenon. This word only occurs in the scene in which Amnon abuses his half-sister Tamar. He pretends to be sick and asks if Tamar could make him some heart-buns. These things most probably were not shaped in a certain way, but rather designed to have someone regain heart, or strength: wellness-cookies.
- The associated denominative verb לבב (libbeb), meaning to make wellness-cookies (2 Samuel 13:6-8 only).