Japanese word people all over the world use in place of “teacher,” but the word “sensei” is much more than that when you look at the kanji.
先 (せん) = previous + 生 (せい) = born
“A person born before you were.” Many dictionaries suggest this is the original meaning of 先生, which makes sense when you look at the kanji. Back in the day, living longer meant (and still implies) having more experience and knowledge.

As taken from https://www.tofugu.com/