Green’s Dictionary of Slang

(N.Z.) a young woman who accompanies a motorbike-riding young social outlaw.

1956		Street Society Christchurch 15: ‘Have you got a motor-bike?’ I asked Bill. ‘Yes. A two stroke [...] [I] Got pinched.’ ‘How?’ ‘Riding over eighty-five down Blank Road with a pillion pussy. Came off once too.’.
1993		R. Yska All Shook Up 144: These cowboys [...] dressed in army greatcoats and peaked train-driver’s caps [...] roared around on bikes [...] attracting girls, known as ‘pillion pussies.’.
1998		Guardian 5 Sept. 35/2: It’s [i.e. New Zealand] a country where a milkbar cowboy has [...] the advantage over a pillion pussy, though neither breed survived the 1950s.
2003		McGill Reed Dict. of N.Z. Sl. 157: pillion pickup/pussy Girl on the back of a motorbike, from the 1950s when her driver was called a milkbar cowboy after his entertainment destination.