Green’s Dictionary of Slang

a generic name for any young male aristocrat.

1895		H.B. Norris [perf. Vesta Tilley] The Piccadilly Johnny with the little glass eye 🎵 He’s very well known is Algy, / As the Piccadilly Johnny with the little glass eye.
1900		Bulletin (Sydney) 28 Apr. 13/3: ‘It would be a blessing if some of the many scantily-employed, high-titled officers [...] were required to see that troops at the front got their letters within reasonable time.’ Ye gods! Would he convert the unemployed Algernon Gussies into dem’d postmasters?
1916		Hull Dly Mail 8 Aug. 3/4: Algy-isms atr the Palace [...] ‘Lord and Lady Algy’ a revival of R.C. Caton’s delightful farce.
1919		E. Dyson ‘In Hospital’ in ‘Hello, Soldier!’ 22: I’m modest, too, / When dividin’ a can of swill / With a Algy boy from the wilds iv Kew. [Ibid.] 26: There are Percies, Algies, ’n’ Claudes I’ve met / Who could take it ’n’ come agen.
1922		E.E. Cummings Enormous Room (1928) 151: ‘Lord Algie’ – a lanky, effeminate, brittle, spotless creature who was en route to becoming an officer and to whose finicky tastes the fat-jowled A. tirelessly pandered for, doubtless, financial considerations.
1936		M. Harrison All the Trees were Green 110: ‘You got to put on a bit of the Algy, you know.’ To elucidate this point, he screwed an imaginary monocle into his eye; drew in his lower jaw, and said ‘Bai Jove’.