Na-adjective (keiyodoshi)
agile; nimble; light (of foot)
捕らえることのできないもの、身軽で素早いもののたとえ、だそうです。It's apparently a metaphor for something uncatchable, light of body and fleet of foot. casual (clothing); light (e.g. luggage)
carefree; with limited responsibility
Also written 身がる【みがる】