gʰeHd-
“to unite, join, be fitting”good, gather, together
Root yielding English good, gather, together from sense of what fits/joins properly.
Discussion
The PIE root *gʰeHd- (to unite, to join, to fit together) is the ancestor of one of English's most fundamental value words: good. The original meaning was not moral but structural — something *gʰeHd- was something that fit, that belonged, that was suitable for its purpose. Goodness, at its etymological root, is a matter of fit.
Old English gōd (good) descends from Proto-Germanic *gōdaz, from PIE *gʰeHd- with a participial suffix: "that which has been joined/fitted." The semantic development from "fitting" to "good" is paralleled in other IE branches: Latin aptus (fit, suitable) carries both the "fitting" and the "good/appropriate" senses, and Greek agathós (good) may share a similar conceptual origin, though the etymological details differ.
The Germanic cognates confirm the root: German gut, Dutch goed, Swedish/Danish god, Icelandic góður, and the extinct Gothic goþs (attested in Wulfila's 4th-century Bible translation, the earliest Germanic text). The remarkable stability of this word — barely changed in three thousand years across all Germanic languages — reflects the conservative inheritance typical of core evaluative vocabulary.
English gather (OE gaderian, "to bring together, to join") is from the same root, preserving the original "unite/join" sense without the evaluative extension. German zusammen (together) shows a different formation from the same conceptual space. The connection between good and gather reveals the root's semantic core: what is gathered together, what fits with the group, what belongs — that is what is good.
The English word together (OE tōgædere) itself contains this root: it is literally "to-gather," to join into one place. The good, the gathered, and the together are all expressions of the same PIE concept: fitness, belonging, union.
A persistent folk etymology connects good with God, but the two words are etymologically unrelated. God (OE god) descends from PIE *ǵʰew- ("to call, to invoke" — the deity as "the one called upon"), while good descends from *gʰeHd- ("to unite, to fit"). The phonetic similarity is coincidental, a product of independent sound changes in the Germanic branch. However, this coincidence has had real cultural consequences: the folk-etymological association of goodness with divinity influenced the reshaping of "God be with ye" into "goodbye" — a contraction driven partly by the unconscious assumption that good and God were the same word.