Meaning of “PaiSen(パイセン)”
“PaiSen(パイセン)” means “Senpai(先輩)”.
“Senpai(先輩)” means kind of senior.
For example, if the person is a colleague and he joined the company earlier, then he is Senpai(先輩).
And also if the person is a student and older than you, he is also Senpai(先輩).
“Senpai(先輩)” is not a slang, but “PaiSen(パイセン)” is a kind of slang in young people.
“PaiSen(パイセン)” is just a word which reverse the order of “Sen Pai(先輩)”.
Like “PaiSen(パイセン)”, reversing the order of the word was popular way for Japanese TV industry people decades ago.
We call it “Gyoukai Yougo(業界用語)”. “Gyoukai(業界)” means industry, “Yougo(用語)” means words.
There were a lot of gyoukai yougo.
TV industry people called “sushi(すし)” “Shi- Su（しーすー）”, called “Model(モデル) “DelMo(デルモ)” decades ago.
But now “PaiSen(パイセン)” is a kind of slang among young people not GyokaiYougo(業界用語).
Opposite word of “Senpai(先輩)” is “Kouhai(後輩)”, but we DON’T call “Kouhai(後輩)” “Haiko”.
Only for “Senpai(先輩)”, we reverse it, I don’t know why.
And be careful, “paisen” is a slang not polite word. So if you are close to the “Senpai”, you can call the “Senpai” “Paisen” friendlily, but if not, it’s rude just a bit.
A: Oh you haven’t finished your work yet?
B: yeah… This is difficult to fix…
A: OK, in this case, you should like this.
B: Wow, amazing Paisen!!