1. (Britain) A small opening built into a dry-stone wall to allow sheep (and hares) to pass through
  2. (humorous, chiefly Greater Boston) A unit of length defined as exactly sixty-seven inches (approximately 1.70 meters), based upon the height of Oliver R. Smoot, who lay on the Harvard Bridge to measure it as an MIT fraternity prank

[Wiktionary, while looking up thirl]