Ha ha!
I think it means wires for fixed installation made up of a handful of thick strands, as opposed to flex which has many thinner strands. Seems to have been popular over here for domestic wiring until the 1970s from what I've seen. Nicer to deal with than the current T+E stuff IMHO.
e.g.
https://uk.rs-online.com/web/p/hooku...E&gclsrc=aw.ds