Green’s Dictionary of Slang

1. an indeterminate period of time.

1864		G.A. Sala Quite Alone III 174: ‘It isn’t a Kathleen Mavouroon farewell, after all,’ he whispered. ‘It won’t be for years, and it won’t be for ever [...] I shall see you often.’.
1903		J. Furphy Such is Life 161: Heaven grant that that parting may be a Kathleen Mavourneen one; and let me have some other class of difficulty to deal with next time.
1936		Irish Parliamentary Debates 1 Jan. 🌐 mr. honan: The man who is not so industrious and who may, for ulterior reasons, refuse to meet his obligations should be treated in the Kathleen Mavourneen manner: ‘It may be years and it may be for ever.’.
1951		S. Hickey Travelled Roads 38: One hawker owed $75 to his supplier [...] and called to tell him he was on a Kathleen Mavourneen (it may be for years, it may be forever) trip to Beirut [AND].
1983		Sydney Morning Herald 20 May 16/2: Two on a lengthening list of the Bowen Basin’s ‘Kathleen Mavourneen’ mines whose development could be for years or could be forever [AND].

2. (Aus.) a prison sentence of indeterminate time.

1907		Lone Hand (Sydney) Sept. 524/2: A ‘Kathleen Mavourneen’ — i.e., the indeterminate sentence which ‘may be for years and may be for ever’.
1910		L. Esson Woman Tamer in Ballades of Old Bohemia (1980) 70: It’s a Kathleen Mavourneen, you know. It may be for years, or it may be for ever. You’d better watch yourself.
1912		Truth (Wellington) 6 Apr. 6/6: ‘A Crook’ Who Needs Reformation. A young man not yet 30 years of age [...] has put up a record qualifying him for the Kathleen Mavourneen stakes.
1919		V. Marshall World of Living Dead (1969) 99: ‘How much longer yer gotter do?’ [...] ‘C176’ chuckled mirthlessly, ‘Mine’s a Kathleen Mavourneen – maybe fer years an’ maybe fer ever.’.
1924		Townsville Daily Bulletin (Aus.) 14 June 13: A life sentence is sometimes alluded to as ‘Kathleen Mavourneen’.
1933		Eve. Post 18 Aug. 9: In the prison vernacular the term ‘habitual criminal’ is known as a [...] ‘Kath,’ videlicet ‘Kathleen Mavourneen,’ because ‘It may be for years, and it may be for ever’.
1943		Baker Popular Dict. Aus. Sl. (2nd edn).
1950		S.J. Baker in Sun. Herald (Sydney) 8 June 9/3: A crook who earns a ‘Kathleen Mavourneen’ (that is, ‘It may be for years and it may be forever . . .’), ‘key,’ ‘twist,’ or ‘The Act’ has been declared an habitual criminal with an indeterminant sentence.
1978		H.C. Baker I Was Listening 41: The judge declared him an ‘habitual criminal’ and gave him a ‘Kathleen Mavourneen’ (‘It may be for years and it may be forever’, as the old song went) [AND].
1988		McGill Dict. of Kiwi Sl. 65/2: kath indefinite term of imprisonment; originally the duration of WWI.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

3. (Aus.) a habitual criminal.

1917		Bulletin (Sydney) I Nov. 24/1: The hawk [...] spells danger and death to many [...] sweet bush singers [...] and so he should get a place with the Kathleen Mavourneens.
1941		Baker Popular Dict. Aus. Sl.
1950		Aus. Police Journal Apr. 116: Kathleen Mavourneen, declared an habitual criminal [AND].

4. (US) a promise; usu. in the context of paying back a loan.

1883		Queenslander (Brisbane) 15 Sept. 11/4: A Debtor [...] declared that the plaintiff knew at the time that it was a Kathleen Mavourneen loan. [...] ‘One of the “it may be for years, and it may be for ecver” sort’.
1906		Queenslander (Brisbane) 7 Apr. 42/5: ‘A Kathleen Mavourneen loan?’ questioned the Court, with a puzzled look. ‘That’s it, Your Lordship’ one of the ‘it may be for years and it may be for ever’ sort.
1917		T.A. Dorgan in Zwilling TAD Lex. (1993) 112: That touch is a Kathleen Mavourneen — Ha — Ha —It may be four years and it may be for ever.
1927		cited in Zwilling TAD Lex. (1993).
1944		D. Runyon Runyon à la Carte 206: So I weed him the thousand and accept his Kathleen Mavourneen, which is a promise to pay that may be for years and may be forever.

5. (Aus.) a pack [presumably refers to the time a vagrant carries his pack].

1922

Smith’s Weekly (Sydney) 28 Jan. 17/4: Swag aliases are [...] ‘Kathleen Mavourneen’ [etc.] [AND].