withered, faded, decayed, calm
蔫 centers on the state of losing vitality: something that has withered, faded, or decayed. The 'calm' sense is a rare extension, perhaps suggesting a quiet stillness that comes after decline.
蔫 combines 艹 (plant) with 焉, which likely contributes the sound. The plant component fits the meaning of withering vegetation. The exact historical development is uncertain.
The plant radical 艹 sits above 焉, like a once-lush plant now drooping and faded. Picture a flower that has lost its color and strength, becoming withered and calm.
For エン, imagine the withered plant at the end of its life: end -> エン, as its vitality fades away.