Hay/heɪ/ Definition 1 of 4Pro Tip 1/3Animal Feed Hay is primarily used to feed domestic farm animals. In the winter, we store extra hay to feed the livestock. Pro Tip 2/3Making Hay 'Make hay' can mean to take advantage of a good situation. We need to make hay while the sun shines and finish our project. Pro Tip 3/3Common Idioms Hay is used in expressions like 'to hit the hay,' meaning to go to sleep. It's been a long day; it's time to hit the hay.