"It's quite nippy"

Have you ever heard this phrase used an about a car that was, in fact, anything other than slow?

(Japanese doesn't count)

...applies to certain motorhomes up in Scotland

All in my opinion of course, but I would use the term for a sportier version of a stereotypically slow hatchback.

I'd say that a Suzuki Swift Sport or a Volkswagen Up Gti is nippy since they are the fastest of their line and are designed to offer more performance without costing much more.

A Fiesta ST, 500 Abarth and the like are 'quick' and obviously a Golf GTI/R and Focus ST/RS are 'fast'. Both lines have other versions which are not slow and offer decent performance for their price.

The term 'nippy' though is often used to describe something both small and slow by non enthusiasts, ie "My daughter has a Toyota Aygo, its nippy". I dont think it is, the car was not designed to offer any sort of performance, it was designed to be cheap and useful, which it is, and nothing more.

As for when it's applied to driving conditions such as traffic, I don't really get that either. Its still a car, you'll have to wait in that queue. If you want to 'nip' through town, a motorcycle far better fits the description.

I like "surprisingly nippy once up to speed" - i.e. once it's finally got to 60, it's doing 60, but it might feel like 70 because it's small, low geared and hasn't got much in the way of soundproofing.

is the correct answer

Also used to refer to a 1.6 Vauxhall Astra or similar, naturally. But never anything genuinely quick, in my experience.

Yes, it's what one of my colleagues said when he showed up to work in a new Nissan GT-R when I asked him what it was like.

Once heard a chap return a BMW S1000RR from a test ride using the phrase to the salesman when asked what he thought of it.
They are quite nippy though, to be fair.

For me it would be a small car with a reasonable engine and a short gearbox. 205 Gti, AX Gti, etc for older cars, Suzuki Swift for newer.

Never going to challenge a sports car but fun to drive and decent acceleration and handling. Feels fast rather than being fast which is a whole combination of factors.

See also 106 Rallye.