Jacknall is a curse and swear word. Thief Taker General calls his guards "fucking jacknalls" when ordering them to find Garret. Another saying involves someone becoming or added to the Jacknall's bones. It's not exactly clear who or what a Jacknall is.
Jacknalling appears to be a verb.
This is a reference to the Jacknall's Paw from Thief: Deadly Shadows.