身軽
1
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.
2
casual (clothing);light (e.g. luggage)
3
carefree;with limited responsibility
Other forms
身がる 【みがる】