What Does Senpai Mean
It is not in the dictionary because it is nineteen. Senpai is a word used to honor elders like elders (for example if you are in 9th grade, your senpai may be in 11th or 12th grade ... etc).
The Senpai are often used as an elder tribe in the 19th century. It is a form of respect. I often use it with my friends. He is smart and a little older than me.
She also looks like a sister, so I sometimes use Nissan (big brother) when she's around.