A compound of 开 (kāi, "to open") + 心 (xīn, "heart/mind"). The literal image is of an "opened heart" — a heart that has expanded with joy. 心 is one of the oldest pictographic characters in Chinese, originally drawn as a depiction of the human heart organ.

开心 is the everyday, warm word for happiness in modern Mandarin — far more colloquially natural than the literary 快乐 (kuàilè). You'll hear it constantly in casual speech, social media, and songs. A common parting wish is "祝你开心！" (Zhù nǐ kāixīn! — "Wishing you happiness!"). Note: 开心 describes a felt, in-the-moment mood, while 快乐 can describe a broader state of contentment or joy, making 快乐 more suitable for formal greetings like birthday wishes.