Green’s Dictionary of Slang

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

1934		D. Runyon ‘Lemon Drop Kid’ in Runyon on Broadway (1954) 370: This is strictly the old ackamarackus.
1946		J. Grierson Grierson on Documentary 74: On examination, it is, of course, the old ackamarackus with a vengeance.
1991		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.
2024		J. Meades Empty Wigs (t/s) 619: ‘[B]’leave me I’m not comin’ the old ackamarackus’.