It's probably a slurred 先輩【せんぱい】. Since there is no context, I am only 95% certain. 先輩 is commonly used to address your senior at school or at work when there is no other appropriate title like 部長. Maybe someone, typically a young school girl, said it in a fawning way.
I think that しぇんぱい shempai is used by mannerless guys with friendly feeling in substitution for 先輩｛せんぱい｝.
This kind of slur is also used like しぇんしぇー shensheh for 先生｛せんせい｝ by them.
Apart from the scenario, さしすせそ is often pronounced like sa shi su she so in the North Kyushu rigion by elders.
The famous singer Tetsuya Takeda is from Fukuoka prefecture in the North Kyushu region. He sings like しぇんしぇい in stead of 先生｛せんせい｝ in his song. He would pronounce the word 先輩 like しぇんぱい.