Green’s Dictionary of Slang

doodle n.¹

a fool, a dull person.

1628		Ford Lover’s Melancholy III i: Vanish, doodles, vanish!
1764		Foote Mayor of Garrat in Works (1799) I 184: Why, doodle! jackanapes ! harkye, who am I?
1775		J. Ash Dict. Eng. Lang.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1832		S. Warren Diary of a Late Physician in Works (1854) III 34: I know it was every word composed by that abominable old addlehead, Dr.—, a doodle that he is!
1890		Music Hall & Theatre Rev. 26 July 11/1: [T]he noble army of Crusaders led by St. MacDoodle the Purist, in their onslaught on Music Halls and other abodes of vice.
1913		Sport (Adelaide) 15 Nov. 7/1: The girl haters said that Scrubby G. is a doodle .
1942		P. Wylie Generation of Vipers 6: Pompous doodles, many of whom hold Heidelburg degrees, thank that about the Germans.
1989		H. Rawson Dict. of Invective (1991) 117: A bumbler or silly person; also the penis. Ding-dong is one of several d words—including ding-a-ling, doodle, dingbat and dork—that combine these two meanings.

In derivatives

doodlesome (adj.)

foolish; infantile.

1829

W. Beckford marginalia in Southey All for Love in Hamilton Palace libraries. Catalogue of [...] the Beckford library (1882) 150: All for pelf rather than all for Love [...] Nothing but the desire for adding to his stock of pence [...] could have induced the Laureate to put forth such a doodlesome publication.