that term seems to float around the "old" car hobby. I see it a lot over on the Shelby Club forum, and I’ve heard that it is a common term.
In general it means that someone used the [assumed not wrecked or rusty] body from a more readily available car [less costly] – Mustang in the case of an early GT350, Alpine in the case of a Tiger – to fix up a poor condition car.

I am unclear on someone admitting to this in writing, as apparently moving the VIN from one car to another is a violation of Federal law. However, in most cases a real expert can figure it out pretty quick… so maybe that is why the seller listed it?