Green’s Dictionary of Slang

1. a large denomination sterling note, £10, £20, £50; thus long-tailed, of more than £5 face value.

1839		H. Brandon Dict. of the Flash or Cant Lang. 163/2: Long-tailed ones – large bank notes.
1842		Era (London) 27 Nov. 10/3: After some discussion [...] the latter placed a ‘long-tailed flimsy’ on the hat of Terry [...] the Slasher’s backer still rmained ‘shifty’.
1859		Hotten Dict. of Modern Sl. etc. 60: LONG-TAILED-ONES, bank notes [...] for a large amount.
1864		Morn. Post (London) 9 Dec. 3/4: Her Majesty’s coin [...] is [known] by no less than 130 distinct slang words, from the ‘brown’ (a half-penny) to ‘flimsies’ or ‘long-tailed ones’.
1870		Hotten Sl. Dict. [as cit. 1859].
1882		Sydney Sl. Dict. (2 edn) 5: Long-tailed Ones - Large bank notes.
1891		J. Bent Criminal Life 271: Long-tailed ... £10 or £20 notes or higher.
1895		C. Crowe Aus. Sl. Dict. [as 1882].
1902		‘The Wasp’ Tales of the Penance Track’ in Bulletin 31 May 31/2: Rajah Riley, having won a small pool the week before, plunged on Revenue to the extent of ten plugs (or ‘long tailors’).
1931		‘English Und. Sl.’ in Variety 8 Apr. n.p.: Long tail ’un — Bank note.
1938		F.D. Sharpe Sharpe of the Flying Squad 331: long-tailed ’uns : Banknotes.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 6: Long tailed ones: Bank of England notes of £5 or upwards.
1977		D. Powis Signs of Crime 191: Long-tai1ed ’un £10 or £20 Bank of England note.

2. (Aus.) an animal (? a kangaroo).

1859

Bell’s Life in Tasmania 9 Aug. 3/5: [of the subject of a hunt] When, however, the long-tailed ’un did get up, he led the field a merry dance.