Green’s Dictionary of Slang

(US campus) a small skullcap worn by freshmen.

1927		W.R. Morse ‘Stanford Expressions’ in AS II:6 276: dink — skull cap for freshmen.
1950		Cornell (University) Daily Sun 18 May 4: Lovers of the old-fashioned dink [W&F].
1996		(con. 1941) P. Fussell Doing Battle 52: Freshman [sic] had special, would-be humiliating headwear, little green flannel ‘beanies’ or ‘dinks’ with the class-year ‘45’ in yellow on the front.