Origins
The word 'mortar' entered Middle English around 1290 from Old French 'mortier,' from Latin 'mortārium,' which denoted a vessel for mixing or pounding — what we would now call a mortar and pestle. The Latin word is of uncertain deeper etymology; some scholars have proposed a connection to PIE *mer- (to rub, to wear away), which would link the vessel to its function of grinding, but this remains speculative. What is clear is that the vessel came first, and the other meanings followed by metonymic and metaphorical extension.
The building material — the paste of lime, sand, and water used to bind bricks and stones — was named by a straightforward metonymy: the stuff mixed in a mortar is called mortar. This transfer of name from container to contents is one of the most common semantic processes in any language (compare 'kettle' in 'put the kettle on,' where 'kettle' means the water inside it). In construction, mortar has been fundamental since ancient times. The Romans developed remarkably durable lime mortars, and their concrete (a form of mortar mixed with aggregate) produced structures like the Pantheon that have stood for two millennia.
The artillery meaning arrived in the sixteenth century, when military engineers noticed that a short, wide-bore cannon designed to lob projectiles in a high parabolic arc bore a resemblance to the upturned bowl of a mortar-and-pestle set. The name stuck: a mortar is a weapon shaped like a mortar. This third meaning has generated its own derivatives — 'mortar fire,' 'mortar round,' 'mortar team' — that now exist independently of both the building material and the grinding bowl.
Figurative Development
The three meanings of 'mortar' illustrate how a single concrete referent (a bowl) can generate an entire family of meanings through metaphor and metonymy. The bowl → the paste mixed in the bowl → the weapon shaped like the bowl. Each step is logical, but the end result — that the same word denotes a kitchen implement, a construction material, and a piece of artillery — can seem baffling without the etymological thread.
German split the meanings more cleanly: 'Mörser' for the grinding bowl and the weapon, 'Mörtel' for the building material. French retains the unified 'mortier' for all three senses, as does Spanish 'mortero.' The cross-linguistic variation shows different languages making different choices about how much semantic breadth a single word can carry.
The 'mortarboard' — the flat-topped cap worn at academic graduations — adds a fourth metaphorical extension. The cap was named for its resemblance to the flat board ('hawk') that masons use to hold mortar, which itself was named for the material it carries. The academic cap thus traces its name, through a chain of four metonymies, all the way back to a Roman mixing bowl.
Legacy
In modern construction, mortar remains indispensable. Portland cement mortar, developed in the nineteenth century, largely replaced traditional lime mortar for structural applications, though lime mortar has seen a revival in conservation work because its flexibility and breathability make it more compatible with historic masonry than rigid cement. The word 'mortar,' which has named a binding paste since at least the thirteenth century, continues to name one of the most fundamental materials in human construction — the substance that holds the wall together, the paste that transforms a pile of stones into a building.