1. What does it mean?
헬린이 blends 헬스, gym training, with 어린이, child. It means someone new to working out or still learning gym routines and equipment. The person being described need not be a child.
Also written or said as hellini
2. The nuance
It can be friendly self-deprecation, including from someone who has trained for a while but still feels inexperienced. It does not describe a medical condition or a fixed fitness level.
3. Examples
“헬린이라 기구 이름부터 배우는 중이야.”
I'm a gym beginner, so I'm starting by learning the equipment names.
“헬린이 때는 모르는 게 많아서 자주 질문했어.”
When I was new to the gym, I had lots to learn and asked questions often.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.