euphrainō

The word comes from two Greek pieces meaning "well" and "the mind or heart," so at its root it means to put someone in a good frame of mind. From Homer's banquets down to the feasts of the Bible, its natural home is the celebration: the shared gladness of eating, drinking, and making merry together. This is not a quiet, private contentment but an outward, public joy that spills over into laughter, food, and song.

The Greek Old Testament made this the standard word for Israel's whole vocabulary of festival joy, especially the command to rejoice before the LORD at the sanctuary feasts, which is why the New Testament writers reach for it whenever a celebration is in view: a father throwing open his house for a returning son, a rich man feasting every day, a heart glad at the hope of resurrection, and heaven shouting in triumph. In one place the apostle Paul turns it around and uses it of one person cheering up another; everywhere else it describes people swept up in celebrating themselves.

What makes this word remarkable is that it passes no judgment of its own. The same verb names holy joy and hollow joy alike, and the biblical writers press that fact hard:

• a father who simply must celebrate a son come back from the dead
• a fool raising a toast to his own overstuffed barns
• a crowd reveling before a golden calf
• all heaven rejoicing as a cruel empire finally falls

The word never tells you whether the joy is right or wrong. What settles that is its object and its direction: what is being celebrated, and with whom. It sits alongside χαίρω, the broad everyday word for joy, and the more exuberant, shouted ἀγαλλιάω, but keeps its own festive, banquet-shaped flavor, matching the Hebrew celebration word שָׂמַח that it so often translates.