The word "精疲力竭" is formed through the parallel combination of two two-character phrases: "精疲" and "力竭". In "精疲", 精 (essence/energy) is modified by 疲 (weary) to indicate exhausted vitality, while in "力竭", 力 (strength) is modified by 竭 (depleted) to denote drained physical power; together, these phrases reinforce each other to convey a state of complete exhaustion.