A boathouse is a building beside a lake, river, or canal where boats are kept. Think of a garage, but for boats: you store the boats, oars, and life jackets, and keep them out of the weather. Some boathouses sit right over the water so you can float a boat in through a door. Rowing clubs, schools, and houses on the water have them. It is a plain, practical word — not slang. Though actually, learners often picture a house that is a boat, a home that floats. That is usually a houseboat. A boathouse stands on land, or on posts by the water; the boats live inside it, and people do not live in it as a rule.