From euthumos; to cheer up, i.e. (intransitively) be cheerful; neuter comparative (adverbially) more cheerfully -- be of good cheer (merry).
see GREEK euthumos
1. transitive, to put in good spirits, gladden, make cheerful (Aeschylus in Plato, de rep. 2, 383 b.). Middle to be of good spirits, to be cheerful, (Xenophon, Plato).
2. intransitive, to be joyful, be of good cheer, of good courage: Acts 27:22, 25; James 5:13. (Euripides, Cycl. 530; Plutarch, de tranquill. anim. 2 and 9.)