NittWitt
Everyone said bathroom when I was young, in Scotland.
I now tend to say convenience.
I've never liked toilet or lavatory and I always thought loo was an English word (as opposed to Scottish).
To go back to underwear, I agree with the OP.
I'd never say underwear if I meant just pants.
Many Scots words used to be French, from the Auld Alliance.
"LOO" was gardyloo, when tenement occupants in Edinburch tipped the "po" out of the window.
Here's lots more Scottish words borrowed from French
<https://scilt.org.uk/Portals/24/Library/MTOT/resources/French%20Scots%20words.pdf>
dinnae fash yersel, they include tartan, caddy, hogmanay.