Green’s Dictionary of Slang

ackamarackus n.

[pig Lat.]

a fraudulent tale, a tall story, nonsense; usu. in phr. old ackamarackus.

[US]D. Runyon ‘Lemon Drop Kid’ in Runyon on Broadway (1954) 370: This is strictly the old ackamarackus.
J. Grierson Grierson on Documentary 74: On examination, it is, of course, the old ackamarackus with a vengeance.
D. Powell Dilys Powell Film Reader 294: The film, in fact, has every opportunity for the oldest ackamarackus. It takes the opportunities – and uses them.