The word "疲惫不堪" is formed by first combining the characters 疲 (meaning weary) and 惫 (meaning exhausted) into the compound 疲惫, which emphasizes severe tiredness; this is then modified by 不堪, where 不 means not and 堪 means to bear, so together 不堪 indicates unbearable, thus the entire term literally conveys a state of exhaustion so extreme it cannot be endured.