The word "坚持不懈" is formed by first combining the characters 坚 (meaning "firm") and 持 (meaning "hold") to create the verb "坚持", meaning "to persevere". This is then directly linked with the characters 不 (meaning "not") and 懈 (meaning "slack") to form the adjective "不懈", meaning "without slackening". The entire compound word structurally pairs "坚持" as the core action with "不懈" as its unceasing modifier, resulting in the literal sense of "persevering without relaxation".