Green’s Dictionary of Slang

crab-apple n.

(US) a sour, ill-tempered person; also attrib.

1897		Bourbon News (Paris, KY) 5 Nov. 4/2: ‘She’s a regular peach,’ said the first chappie. [...] ‘She’s a crab-apple to me’.
1898		Kansas City Jrnl (MO) 27 Feb. 8/5: Miss Mellish [...] became an embittered old maid. Dear old soul, she was a perfect crab apple.
1914		C. Holme Lonely Plough (1931) 90: Never saw such a jaundiced old crab-apple in my life!
1929		S.J. Perelman in Marschall That Old Gang o’ Mine (1984) 64: Here is the very deuce of a yarn sent in from the sticks by some crab-apple Eddie Cantor in Ioway.

SE in slang uses

In compounds

crab-apple two-step (n.) [the result of eating sour fruit]

(US) diarrhoea.

1967		in DARE.
2006		Sleaze Jun–Jul. 🌐 ‘I told them coppers I was touching cloth, but they told me I’d just have to hold on,’ pensioner Harry Dunton told reporters, as he danced the crab-apple two-step whilst queuing outside the toilets at Waterloo station.