Meaning of “SutaMen(スタメン)”

“SutaMen(スタメン)” means “starting line-up”.
We call “starting line-up” “Start member” as Japanese English.
“Suta(スタ)” is an abbreviation for “Start(スタート)”,
“Men(メン)” is an abbreviation for “Member(メンバー)”.

Example Conversation

A: You belonged to the baseball club in high school?
B: Yeah, but I was a substitute player. How about you?
A: Baseball club too! Basically I was “sutamen(スタメン)”, but our team was really weak and had no chance to win in the official matches.

A: お前、野球部だったの？
B: うん、補欠だったけどね。お前は？
A: 俺も野球部！基本はスタメンだったけど、試合で勝てない弱小野球部だったよ。