Green’s Dictionary of Slang

clothes-prop n.

[see Williams for 17C fig. use of prop, penis]

1. the erect penis.

[UK]Farmer & Henley Sl. and Its Analogues.

2. a silly, empty-headed person.

[UK]A.N. Lyons Hookey 142: I turned round to make a few caustic remarks to them grinnin’ clothes-props.

3. see clothes-peg n.1