Sino-Korean: 約束 (약속). 約 (약) means "to bind/restrain" and 束 (속) means "bundle/bind together." The compound captures the idea of two people tying themselves to an agreement.

In Korean culture, keeping 약속 carries significant social weight — cancelling plans last-minute, especially with elders or seniors, is considered quite rude. The word covers both casual meetups ("let's hang out") and solemn promises, so context matters. Watch out for the false friend: this word does NOT mean "medicine" (that's 약, 藥 — a different hanja entirely).