vt., obs. (alt. 'jfcl') To cancel or annul something. "Why don't you jfcl that out?" The fastest do-nothing instruction on older models of the PDP-10 happened to be JFCL, which stands for "Jump if Flag set and then CLear the flag"; this does something useful, but is a very fast no-operation if no flag is specified. Geoff Goodfellow, one of the jargon-1 co-authors, has long had JFCL on the license plate of his BMW.
Usage: rare except among old-time PDP-10 hackers.