Amsterdam is named for a piece of civil engineering.
Etymonline says the name refers to a dam built on the Amstel river. Britannica says early residents built dikes on both sides of the river to protect themselves from floods. About 1270 they built a dam between those dikes. Wikipedia gives a wider estimate, saying the dam was probably built between 1264 and 1275.
The name first appears in writing in 1275. Britannica says Count Floris V of Holland granted the people living by the dam a toll privilege that year, and calls the document the first to mention the name. The document speaks of "homines manentes apud Amestelledamme," which Britannica renders "people living near the Amstel dam." Wikipedia's History of Amsterdam dates the document to 27 October 1275 and calls it the oldest known record of the name.
The river's own name is older. Etymonline says it is traditionally explained as Germanic elements meaning "current" and "place," and presents that as an explanation, not a settled fact. Wikipedia says the older form Amestelle likely meant "watery area." The two glosses differ in wording, but both describe a place defined by water.
The spelling then wore down. Wikipedia's History of Amsterdam says the name had become Aemsterdam by 1327. It reports 18 different spellings in charters between 1275 and 1596, with the modern form recorded in 1440.
The dam also gave the city its main square. Wikipedia says Dam Square is named after the original dam and holds the Royal Palace and the National Monument.
Amsterdam is not the only Dutch city built on this pattern. Etymonline says dam appears often in Dutch place names such as Rotterdam and Edam, reflecting Holland's geography. Wikipedia says Rotterdam is named for the Rotte, a fen stream, plus a dam built on it in the 1260s, though it elsewhere dates the dam to 1270.
The name also crossed the Atlantic. Etymonline notes that New York was formerly New Amsterdam, renamed in 1664 after the British acquisition.