motte

The etymology of moat is a strange one. The word appears to derive from Old French motte, meaning a mound, hillock, or raised platform. In the motte-and-bailey castle design — the earliest Norman fortification — the motte was the raised mound on which the keep was built. The bailey was the enclosed courtyard below. The word described the highest point of the castle, not the lowest.

How motte became moat — how a mound became a ditch — is debated. One theory is that the ditch around the motte's base transferred its name upward (or downward). When you dug earth to build the motte, you created a ditch. The ditch was the motte's negative, its absence. The word may have attached to the space left behind. Another theory suggests confusion between Old French motte and a word related to Middle English mote (a particle, a small piece), though this connection is tenuous.

By the thirteenth century, moat in English clearly meant the water-filled ditch around a castle. The mound meaning had disappeared in English. The moat was a defensive feature — crossing it under fire was dangerous, undermining walls behind it was difficult, and the water prevented direct assault on the castle's foundation. Some moats were dry ditches. The water was not required. The depth was the defense.

Modern English uses moat almost exclusively as a metaphor. Warren Buffett popularized the 'economic moat' — the competitive advantage that protects a company from rivals the way a moat protects a castle from attackers. Brand strength, patents, network effects — these are moats in business language. The Old French word for a mound, which became the English word for a ditch, has become the business word for anything that keeps competitors out.