"blank or missing portion in a manuscript," 1660s, from Latin lacuna "hole, pit," figuratively "a gap, void, want," diminutive of lacus "pond, lake; hollow, opening" (see lake (n.1)). The Latin plural is lacunae. The word has also been used in English from c. 1700 in the literal Latin sense in anatomy, zoology, botany. The adjectival forms have somewhat sorted themselves: Mathematics tends to use lacunary (1857), natural history lacunose (1816), and lacunar (n.) is used in architecture of paneled ceilings (1690s), so called for their sunken compartments. Leaving lacunal (1846) for the manuscript sense.