The Greek adjective leios (ฮปฮตแฟฮฟฯ) means smooth, level, flat, or plain. It appears in Luke 3:5 in the citation from Isaiah 40:4 that frames John the Baptist's mission: "the rough ways shall be made smooth (leias)\” โ a metaphor of the preparation required before the coming of the Lord.
The Isaiah 40 passage quoted in all four Gospels is a comprehensive description of eschatological highway construction: valleys filled, mountains leveled, rough paths made leios. This is the language of divine preparation โ the King's arrival requires transformation of the landscape before Him. John the Baptist is the herald whose preaching reshapes the terrain of human hearts.
The "rough ways" made leios is a metaphor of repentance โ the jagged, resistant terrain of human pride and sin must be smoothed for the King to travel. Zechariah 4:7 promises: "What are you, O great mountain? Before Zerubbabel you shall become a plain (leios).\” The Spirit of God makes the impossible path navigable. Every obstacle of sin that seemed immovable becomes flat before the advancing Kingdom.