I have heard that 'Loo' comes from 'Garde l'eau' or 'look out for water,' shouted when emptying a chamber pot into the street, -in the old days, of course! Not sure that I believe it.

Q, "once a word is labelled as dialect or as a colloquialism, it is dropped from use"???   That's where they come from in the first place! Used by some 'in' group, picked up and promoted by talking heads on TV.