The log which, if removed, would free up the whole logjam.
The key issue or problem, which if (re)solved, would make the current task easy to complete. The issue around which the whole problem revolves.
1932, Homer L. Dodge, “The Improvement of College Teaching”, in The Journal of Higher Education, Volume 3, Number 9 (December 1932), Ohio State University Press, page 485:
In a log jam, where everything is at cross-purposes, there is somewhere a key log causing the trouble. The key log in the educational jam is the department, and the things which have caused it to make so much trouble are narrow departmental ambition and excessive striving for reputation through the publication of research.
1938, Col. Leonard P. Ayres, quoted in ""Cheapskate" Counterpoint", in Time magazine, Monday, Jan. 10, 1938
Present conditions are "a crisis of confidence. . . . Electric utilities are the key log."
