capacity (n.)

early 15c., "ability to contain; size, extent;" also "ability" in a legal, moral, or intellectual sense, from Old French capacité "ability to hold" (15c.), from Latin capacitatem (nominative capacitas) "breadth, capacity, capability of holding much," noun of state from capax (genitive capacis) "able to hold much," from capere "to take," from PIE root *kap- "to grasp."

Sense of "power to store electricity" is from 1777; industrial sense of "ability to produce" is from 1931. Meaning "power of containing a certain quantity" is from 1885, hence "largest audience a place can hold" (1908).