To double up. The term is used in logging, when two logs, jammed in the fashion of a half-open knife, obstruct the others.
1897. One of their amusements in camp at that time was to throw stones and chips past one another’s heads, and raise a laugh at the active dodging and bending the body low, or “jack-knifing,” as the men called it.—H. Porter, ‘Campaigning with Grant,’ p. 140. (N.E.D.)
