He played first baseman in high school baseball.
一塁手が難しいゴロをさばいた。
The first baseman handled a difficult grounder.
二塁手 covers second base, while 一塁手 covers first base—easy to mix up because both end in 塁手.
三塁手 is the third baseman while 一塁手 is the first baseman, so the two are easily mixed up since they share the same 塁手 structure.
Compound of 一 (one), 塁 (base), and 手 (player).