There are some words in Biblical Hebrew that are difficult to interpret because they only appear once in the entire Tanakh. We've discussed plenty of those. However, there are other roots that are so common, and have such variety of meaning, that it can be just as difficult to pin down the "main" sense (if there even is one.) The root חסד is certainly one of those cases.
Its two main forms appear frequently: חֶסֶד hesed (246 times) and חָסִיד hasid (32 times). But what do they mean? Hesed can be easily defined as "kindness" (or an act of kindness), "grace", or "mercy." The related hasid is either an adjective, hesed-like, or a noun, "one who does hesed." But that doesn't make its translation any simpler - it can mean (one who is) pious, devout/devoted or kind.
So lets look at some different explanations of these words and how scholars have tried to interpret them.
Klein has both words representing kindness. He defines hesed in this order:
1 kindness, goodness, mercy. 2 affection. 3 lovely appearance.
And hasid, according to him, has a similar development. In Biblical Hebrew it means "kind, benevolent", and only in Modern Hebrew does it gain the sense of "pious, godly, devout."
The BDB entry (note the new Sefaria BDB resource!) goes further than Klein. They also have the root starting with "kindness", but note that both words can refer to piety in the Tanakh as well (e.g, hesed - Yeshaya 57:1; hasid - Tehilim 4:4).
Gesenius says the root has a different original meaning: "to love, desire." This "desire" comes to mean "zeal" or "love" for anyone - expressing itself in kindness or mercy. In other contexts, it can reflect piety (towards God) or the grace of God toward humans. That sense of grace is expanded, in some cases, to beauty in general (as in Esther 2:9,17).
The Ben Yehuda dictionary entry for hesed begins with the translation "grace" and explains it as something "beyond the requirement of the law, not done out of obligation but because of love." In fact, this is the only translation offered by Ben Yehuda. As far as hasid, he initially defines it as "one who acts with hesed," then "one who acts with tzedek," and only in the third definition offers the translation "pious" (for which he does provide biblical sources.)
Steinberg goes in a different direction. He says that the root חסד means "diligent." When diligent in the positive sense, that can lead to generosity, kindness, love, and devotion. Perhaps this sense of devotion can explain how hasid came to mean someone devoted to God (i.e., devout, pious) more than just someone who is kind.
From the sense of "pious ones", the term was adopted by those opposing Hellenistic Jews in the Second Temple period, and later following this, by an Ashkenazi religious community in the 12th and 13th centuries, and the Jewish spiritual movement begun in 18th century Europe. Today, in secular Hebrew, a hasid, can be a devotee or follower of any movement or individual.
There are two related terms to חסד that we have not yet discussed. One is the surprising use of hesed in a negative context. It is not common - only appearing in Vayikra 20:17 where it means "disgrace" and Mishlei 14:34, where it means "reproach" (as well as in the verb form in Mishlei 25:10). How did it obtain this opposite meaning to all else that we've seen?
Klein gives two possible answers. One is that they come from different roots. He writes for this meaning of חסד:
he insulted (a hapax legomenon in the Bible, occurring Pr. 25:10). [Aram. חֲסַד (= was put to shame), Aram.-Syr. חַסֵּד (= he reproached, reviled), Aram. חִסְדָּא (= shame), Syr. חֶסֽדָּא (= shame, reproach, ignominy), Arab. ḥasada (= he envied). Some scholars connect Arab. ḥasada with MH חָשַׁד (= he suspected). See חשׁד. See also חֶסֶד ᴵᴵ.
But he also offers the suggestion that this is a case where one root can contain two opposite meanings. For his second definition of hesed, he notes:
According to some scholars חֶסֶד ᴵᴵ and חֶסֶד ᴵ are of the same origin. For the ambivalence of meaning cp. בֵּרַךְ (= he cursed), which is ult. identical with בֵּרַךְ (= he blessed)
Perhaps this is a case of a contronym, which we have discussed several times. The BDB, for example, writes that the same "eager zeal or desire" which led to kindness, can also lead to envy, shame, and reproach.
The other word which may be related is the Hebrew word for "stork", חֲסִידָה - hasida. In his entry, he defines it as:
lit.: ‘the pious bird’; so called in allusion to its love for its young
He notes that the Latin word for stork, pietaticultrix, had the same meaning - representing its dedication to both its young and its parents. When the hasida is mentioned in Vayikra 11:19, Rashi, quoting Hullin 63a, comments:
Why is it called hasida? Because it acts kindly with its fellows in respect to food.
However, a question remains: if the stork acts with hesed, why is it listed as a non-kosher bird? An answer offered in the name of various Chassidic (!) rebbes is that the stork is devoted only to its own kind. That may be a sign of piety, but it is not a sign of kindness - and so the stork is not kosher. In our review of the various meanings of hesed, this is a very important lesson to remember.