Green’s Dictionary of Slang

(US, usu. juv.) foolish, silly, eccentric.

1981		Friedwald & Beck Warner bros. Cartoons 217: Sylvester and Sam (the doofy orange cat with the Frank Fontaine voice) resolve to chase Tweety no more.
1987		G.A. Fine With the Boys 177: The insult category consisted of [...] : gaywad, bite me, doofy, dork, mutt.
1993		(con. 1969) N.L. Russell Suicide Charlie 29: None of us wanted to appear doofy in front of so many other young men.
2011		G. Hayward Corruption Officer [ebk] cap. 6: They were not impressed. One inmate said, ‘Look at this doofy mother fucker’.