Definition from Wiktionary, the free dictionary
umstroke (plural umstrokes)
- (obsolete) The circumference or periphery of a circle.
- (archaic) A boundary line, especially one at the outermost limit; an extreme edge.
1608, Thomas Fuller, Pisgah Sight of Palestine:
- Such towns as stand, as one may say, on tiptoes on the very umstroke, or on any part of the utmost line of a map […]