Green’s Dictionary of Slang

an outstanding person or thing, a prodigy.

1821

Egan Life in London (1869) 258: Sir, you vou’d like to see a bit of fun to night vith this here little phenomy Monkey. [Ibid.] 260: I say, governor, how long are ve to be kept in this here rookery before you give us a sight of this Phenomony.