From ThresholdRPG Wiki

Your tummy statistic is how much you can eat before you become full. Your max tummy value increases as you level. While you cannot see your actual tummy number, you can monitor its state via the Summary command or via the Mudlet gui.

Every time you eat food, your tummy value increases until you can no longer eat. Your tummy value will automatically reduce over time as your body digests it. Some abilities and other magical means, such as potions, can increase this digestion rate or, in full or in part, wipe it completely.