noun
Archaic term for a spy or undercover agent, sometimes associated with ninja in historical contexts. Not used in contemporary Japanese; modern equivalents include スパイ or 間諜 (かんちょう).
戦国時代には、細作が敵陣に潜入して情報を集めた。
In the Warring States period, spies infiltrated enemy camps to gather intelligence.
The exact derivation is uncertain; the kanji 細 (fine, detailed) and 作 (make, work) may suggest 'one who does detailed work', fitting the covert nature of espionage.