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
彼は一所懸命に働いて家族を養った。
He worked with all his might to support his family.
I studied desperately to pass the exam.
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.
Medieval warriors defended their territories with the spirit of devoting themselves entirely to that one place.