staunch devotion to one's master; single-hearted loyalty
A yojijukugo describing unwavering, wholehearted loyalty to a lord or master, often with a classical or literary flavor.
He was known as a samurai of staunch devotion to his lord.
The lord was deeply moved by his retainer's single-hearted loyalty.
忠義 is the general concept of loyalty or devotion, while 忠義一徹 emphasizes an unwavering, single-minded quality of that loyalty.
一徹 means stubbornness or single-mindedness on its own; 忠義一徹 specifically applies that single-mindedness to loyalty toward a master.
A yojijukugo combining 忠義 (loyalty) and 一徹 (single-minded, stubborn). The exact historical origin is uncertain, but it follows the pattern of classical four-character compounds expressing moral ideals.