Green’s Dictionary of Slang

(US gambling) in craps, the point of nine.

1898		St Paul Globe (Minn.) 28 Aug. 13/5: Each point on the dice has a certain name, which is known to every southern ‘nigger’ [...] Nine is [...] ‘ninety days’.
1911		Howitzer (US Milit. Academy) 177: Ninety days—seven come eleven! [HDAS].
1935		A.J. Pollock Und. Speaks.
1961		J. Scarne Complete Guide to Gambling.
1987		in T.L. Clark Dict. Gambling and Gaming n.p.: You can hear any oldtimer use ninety days for the point nine ...That was a standard [jail] sentence for getting caught running a game [HDAS].
2004		‘Animated Dominoes, Dice’ at Old and Sold 🌐 Some of the best-known nicknames in dice are: [...] Ninety days, Nina: total of nine.

(US) an inexperienced employee or one employed for temporary work.

1931		Shelby & Stoney Po’ Buckra 159: I leaned more about bein’ a soldier from watchin’ the way he walked around [...] than from anything that ninety-day wonder, Thompson, ever taught me.
1962		(con. 1940s) G. Mandel Wax Boom 21: Big shot ninety-day wonder. The goddamn phoney.
1962		(con. WWII) J.O. Killens And Then We Heard The Thunder (1964) 36: Who the hell do you think you are? [...] You you goddam ninety-day wonder.
1975		C.W. Smith Country Music 25: Bobby Joe had grown up hearing the phrase ‘ninety-day wonder’ applied not only to second lieutenants but likewise to anyone in the oil industry with a degree [HDAS].
2000		‘David Lipscomb And His New Church’ in Plumbline V:5 Dec. 🌐 Even a ninety-day wonder could discern the facts in this matter.