Double Play
noun, verbal noun
A join kill is when in one play you get two kills (outs) in baseball, making it a double play.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
「ゲッツー」というのは、併殺を意味する和製英語です。
"Get two" is a Japanese word derived from English meaning double play.