Green’s Dictionary of Slang

(US) an outstanding person or thing, a prodigy.

1887		World (N.Y.) 14 Aug. 3/5: Hunting for ‘moss-agate’ pitchers is one of the ills that baseball managerial flesh is heir to [...] He had been Informed that a wonder in the twirling line resided in Harlem and early yesterday morning he was in town to sign ‘the wonder.’ But the ‘phenom’ did not materialize, and Denny was in despair.
1896		W.C. Gore Student Sl. in Cohen (1997) 7: phenom n. Phenomenon. Something out of the ordinary.
1902		Laurens Advertiser (SC) 15 Jan. 4/1: Either this ‘phenom’ is young, or it was picked mighty green.
1909		W. Irwin Confessions of a Con Man 40: They’ve found a young phenom.
1912		Ade Knocking the Neighbors 70: He slapped old Ready Money on the Back and told him his Daughter was a Phenom.
1919		D. Runyon ‘A Tale of two Fists’ XIX in Pittsburgh Press (PA) 25 May 25/1: [They] could not have recalled the name of Bill’s phenom two hours later .
1952		Lait & Mortimer USA Confidential 106: Cummings first emerged as a phenom while District Attorney of Fairfield County.
1967		R. Daley Only a Game 95: ‘Did Conroy [i.e. a new star recruit] worry you?’ ‘Every year I look over these phenoms. I ain’t seen nobody yet’.
1970		L. Schecter Polo Grounds 31: [T]he two rookies Casey Stengel chose to tout to the world as young pheenoms.
1991		J.B. Holway Josh & Satch 87: [T]he 17-year-old Iowa phenom, Bobby Feller, who had whiffed 17 Cards in one exhibition.
2003		www.pulpgen.com [blurb] Thursday in top form with a yarn about a fight promoter and his mick-warwhoop (Irish-Indian) phenom.
2012		R.A. Dickey Wherever I Wind Up 2: The word ‘phenom’ [...] —a word that anoints you as the embodiment of hope, someone whose youthful gifts are going to bring joy and victories for years to come.
2020		B. Dempski et al. Dalko 10: His coach [...] considered Steve a ‘phenom’ from the first time he saw him pitch.