The word 朝气蓬勃 is formed by directly combining the two-character unit 朝气, meaning "morning air" and metaphorically conveying freshness and vitality, with the two-character unit 蓬勃, meaning "vigorous" or "flourishing", to create a four-character compound where the latter intensifies the former, resulting in an idiom that denotes a state of full energy and liveliness.