Green’s Dictionary of Slang

1. (US campus) a student who does nothing but study.

1927		W.R. Morse ‘Stanford Expressions’ in AS II:6 276: dryball — a student devoting all his time to study.
1960, 1975		Wentworth & Flexner DAS.

2. (US) a puritan, a moralist.

1974

G. Radano Stories Cops Only Tell Each Other 165: Our captain was a wop. Not a bad guy but a dry-ball. I ask him if there’s any chance of getting into Civil Defense—a day job. He says [...] drop dead.

3. (Aus./US) in pl. an insult, the inference being that the target is impotent or celibate, also attrib.

1991		R.G. Barrett Between the Devlin 121: ‘Unlike you, dry-balls, I have been getting my end in’.
2021		T. Pluck Boy from County Hell 90: ‘They like the uniform, and their dry-balls husbands can’t do shit about it’.