adverb, na-adjective, noun
with all one's might; desperately; very hard
Used for doing something with maximum effort, often in a desperate or earnest manner. Frequently interchangeable with 一生懸命 in modern usage, though purists may distinguish them.
See also: 一生懸命
He worked with all his might to support his family.
試験に合格するために一所懸命勉強した。
I studied desperately to pass the exam.
noun
devoting oneself to defending one's territory (of a samurai)
Historical term referring to a samurai's single-minded devotion to protecting the land granted by their lord. Rare in modern conversation except in historical contexts.
中世の武士は一所懸命の精神で領地を守った。
Medieval warriors defended their territories with the spirit of devoting themselves entirely to that one place.