dill pickle, n. — Green’s Dictionary of Slang

dill pickle n.

[despite appearances, the ref. is to the apparent absurdity of a gherkin (? its phallic resemblance) thus cf. wally n.² (1), and not to the later Aus./N.Z. dill n.¹]

(US) a fool; also attrib.

1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 47: [heading] Shortribs Enlivens Court Proceedings by Calling Beany A ‘Dill Pickle.’.
1913		‘Lord Ballyrot in Slangland’ in Tacoma Times (WA) 12 July 4/4: Why do your dillpickle dreadnaughts always try to kick me off the surface of the drink, huh?