1. What does it mean?
새내기 is a friendly word for someone new to a group or activity. On campus it commonly means a freshman or first-year student learning their way around university life.
Also written or said as saenaegi
2. The nuance
It is not restricted to a particular age. It can also describe a new employee or hobby beginner, and often sounds warmer than a blunt label for inexperience.
3. Examples
“새내기 때는 강의실 찾는 것도 어려웠어.”
When I was a freshman, even finding the classroom was difficult.
“동아리 새내기들에게 활동 방식을 설명했어.”
We explained how the club works to its new members.
Sources
National Institute of Korean Language
References support the meaning and usage. Example sentences are written for NoCapWiki.