When it comes to describing someone who possesses an unwavering spirit and boundless energy, several synonyms for the word "infatigable" come to mind. One such synonym is "unflagging", which conveys the idea of tireless dedication and perseverance. Another alternative is "indefatigable", emphasizing a relentless drive that knows no limits or weariness. A similar term, "untiring", emphasizes the ability to persist without succumbing to exhaustion. Additionally, "unwearied" encapsulates the notion of continuous action without showing signs of fatigue. These synonyms showcase the remarkable endurance and resilience that characterize individuals who can be described as "infatigable."