Green’s Dictionary of Slang

muldoon n.

1. (US, later use US black) a police officer [a typical Irish name; US police were stereotypically Irish].

[US]Current Sl. V:2.

2. (US) the truth, the real thing; something or someone dependable [abbr. ‘the solid Muldoon’, popularized by ‘Muldoon the Solid Man’, a vaudeville song by Harrigan and Hart (1874)].

[[US] ‘English Sl.’ in Eve. Telegram (N.Y.) 9 Dec. 1/5: Let us present a few specimens:– [...] ‘Like Muldoon, he is a solid man.’].
[US]‘Bill Nye’ Bill Nye and Boomerang 182: Go to your husband [...] Lean on him. He will be your solid muldoon.
[UK]Kipling in Soldiers Three [story title] The Solid Muldoon.
[US](con. 1890s) S.H. Adams Tenderloin 170: The madams ain’t givin’ us the solid Muldoon on this thing.

3. (US Und.) a stolen credit card.

H. Hill in Pileggi Wise Guy (2001) 20: ‘Paulie called stolen credit cards “Muldoons,” and he always said that liquor tastes better on a Muldoon’.

4. a run-of-the-mill individual.

[US]Simon & Burns ‘Time After Time’ Wire ser. 3 ep. 1 [TV script] I don’t want to be just another Muldoon, like Dominic DePietro, warming the back bench.