When a liquid gets hot enough, it turns into a gas. When a liquid is hot enough that it's turning into a gas, it's said to be boiling. Immersing raw food products in boiling liquid is a common cooking technique, food cooked in this way is said to be boiled. A peanut is the edible seed, or pulse, of a peanut plant, a tropical legume. Ergo a boiled peanut is a peanut which has been boiled.