Originally 'to bestow grace from above' — used for an emperor pardoning a criminal, a deity granting blessings, or a Christian God showing grace. In modern speech it survives in pleading-for-mercy clichés (
求…
开恩) and ironic everyday use (
老板开恩 'the boss took pity'). Note the apostrophe in pinyin: kāi + ēn → kāi'ēn, since 'en' is a separate syllable.