Green’s Dictionary of Slang

dud n.2

[? dud n.1 , thence rags and thus one who dresses in them, esp. a dudman, a scarecrow]

1. [early 19C; 20C+] of a person, a failure, an incompetent, a weakling, a bore.

2. [20C+] anything that lit. or fig. ‘does not work’.

3. [20C+] of a thing or event, a failure, a disappointment, a ‘flop’.

In compounds

dudhead (n.) [-head sfx (1)]

[1960s+] (US) an idiot; also as adj.