Green’s Dictionary of Slang

(US) a mean, ill-tempered individual.

1966–7		in DARE.
1990		J. Brennan Treasured Times with Five- To Ten-Year Olds 121: I’m a little crankpot, raging mad, / [...] / When I get so angry, it’s really bad.
2002		Xtra! 30 May 🌐 The film focuses on the rather awful manner in which Brian’s mother treats her son. She’s been a real crankpot ever since her husband left her when Brian was a boy.