Green’s Dictionary of Slang

a man who is either not very intelligent (‘nothing up top’) or endowed with large genitals (‘it’s all down below’).

1924		(con. 1880–1924) F.J. Wilstach Anecdota erótica 38: I call my sweetie bungalow, because there is nothing upstairs.
1926		Maines & Grant Wise-crack Dict. 6/1: Bungalow – one without an upper story.
1945		Yorks. Post 25 Apr. 2/4: Williams [...] wrote some verses about the battery in which he served [...] one of the gunners did not appreciate his own appearance in the poem under the name Bungalow Bill.
1975		A. Bleasdale Scully 24: Everyone calls him ‘Bungalow Head’ ’cos he’s got nothing upstairs.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 123: Jim — this is Bungalow [...] Know why he’s called Bungalow? Cos he’s got nuffin upstairs, has he?
1997		Share Slanguage.