alarm clock, n. — Green’s Dictionary of Slang

alarm clock n.

[a person who keeps one, in real or fig. uses, from ‘falling asleep’]

1. (US) a worrier, a nag.

1922

J. Tully Emmett Lawler 120: Why, Auntie! You awful alarm clock. I never felt better.

2. (US campus) a chaperon.

1922		Morn. Tulsa Dly World (OK) 7 May 29/7: Alarm clock - A chaperon.
1927		M.C. McPhee ‘College Sl.’ in AS III:2 132: The offical chaperon at a party is referred to as ‘an alarm clock’.
1938		Eve. Post (Wellington) 25 Jan. 8/8: Modern Americanisms [...] Naturally you would sooner go to a ‘hob-nail hop’ [unaccompanied] by an ‘alarm clock’.
1949		Monteleone Criminal Sl. (rev. edn).

3. (US black) a college professor.

1947

M.H. Boulware Jive and Sl.