"mixture of cement, material used (in building) for binding together stones or bricks," mid-13c., from Old French mortier "builder's mortar, plaster; bowl for mixing" (13c.) and directly from Latin mortarium "mortar, mixture of lime and sand," also "crushed drugs," which probably is the same word as mortarium "bowl for mixing or pounding" (see mortar (n.2)), with the sense transferred from the bowl to the material prepared in it. Dutch mortel, German Mörtel are from Latin or French.
"bowl for pounding, vessel in which substances are beaten to powder by means of a pestle," c. 1200, from Old French mortier "bowl; builder's mortar" and directly from Latin mortarium "bowl for mixing or pounding," also used of the material prepared in it, a word of unknown origin as it is impossible now to determine which sense was original. Watkins says probably from PIE root *mer- "to rub away, harm;" de Vaan finds this plausible. Late Old English had mortere, from the same Latin source, which might also be a source of the modern word. German Mörser also is from Latin.
"short cannon, ordnance piece short in proportion to the size of its bore," fired at a high angle and meant to secure a vertical fall of the projectile, 1620s, originally mortar-piece (1550s), from French mortier "short cannon," in Old French, "bowl for mixing or pounding" (see mortar (n.2)). So called for its shape.