From Greek ho amnos tou Theou — "the lamb, the one of God." John the Baptist's declaration in John 1:29 ties together three OT streams: the Passover lamb whose blood caused the destroyer to pass over (Exodus 12); the daily tamid lamb sacrificed morning and evening in the temple (Exodus 29:38-42); and the suffering servant "led like a lamb to the slaughter" in Isaiah 53:7. Webster 1828 notes lamb figuratively of a person of meek, gentle disposition.
To call Jesus the Lamb of God is to say that He is the sacrifice God Himself provides. Abraham told Isaac "God will provide for Himself the lamb" (Genesis 22:8); two thousand years later, John the Baptist points and says "Behold, the Lamb of God, who takes away the sin of the world!" The Passover imagery is precise: His bones are not broken (John 19:36, Exodus 12:46); His blood marks the doorposts of believing hearts so that wrath passes over; He is eaten by faith, giving life to those inside the house. In Revelation the slain Lamb is the one worthy to open the scroll (Rev 5) — the wrath of the Lamb is the most terrifying phrase in the Bible (Rev 6:16), because only the meek sacrifice could righteously judge. The Lion is a Lamb; the Lamb is a Lion.