fatigue
憊 centers on deep exhaustion: a state of extreme fatigue where one's energy is completely drained.
憊 combines 備 (prepare, equip) with 心 (heart/mind), suggesting a state where the heart or mind is completely used up, leading to fatigue. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The top 備 looks like a person preparing equipment, and the bottom 心 is the heart. Imagine preparing so much that your heart gives out from exhaustion.
For ハイ, picture a tired person letting out a heavy sigh: 'hai...' The sigh sounds like ハイ and captures the feeling of deep fatigue.