A houseboat is a type of boat; a boathouse is a type of house.
This illustrates a common pattern in English morphology: the rightmost part of a compound (houseboat) is usually the ‘head’. In other words it’s the centre or larger category, functionally equivalent to the overall compound, and what precedes it (houseboat) modifies or specifies it. So we say English is ‘right-headed’.
But the semantic relationship between the parts can’t be inferred automatically from their arrangement, as this charming/disarming Bizarro cartoon by Dan Piraro shows:
Right-headedness is a feature of Germanic languages. Romance languages tend to reverse the order: chaise longue is a type of chaise, lingua franca a type of lingua. Either way, when a compound includes the head it is called endocentric – the centre is internal. In exocentric compounds the head is missing or external: a bigmouth is not a type of mouth and an egghead is not a type of head – both refer to people.
Editor and historical linguist Brianne Hughes studies a remarkable subset of exocentric compounds called agentive and instrumental exocentric verb-noun (V-N) compounds. Mercifully, and memorably, she calls them cutthroat compounds, or cutthroats for short. These are rare in English word-formation but have a long and colourful history and constitute a very interesting category.
Cutthroat compounds name things or people by describing what they do. A cutthroat cuts throats, a telltale tells tales, a wagtail wags its tail, a killjoy kills joy, a scarecrow scares crows, a turncoat turns their coat, rotgut rots the gut, a pickpocket picks pockets, a sawbones saws bones (one of the few plural by default), and breakfast – lest you miss its etymology, hidden in plain sight – breaks a fast. The verb is always transitive, the noun its direct object.