第だい一いっ線せん

noun

front line; forefront

Refers to the very front of a battlefield or, by extension, the leading edge of any field or activity. Often used in phrases like 第一線で活躍する (to be active on the front lines).

• 彼かれは長なが年ねん、医い療りょうの第だい一いっ線せんで働はたらいてきた。

  He has worked on the front lines of medical care for many years.