The word "locum" appears in Danish - spelling: lokum - referring to "the (secret) place" and is thus a euphemism for a latrine, especially of the more primitive kind. The word has been in use since the 1740's and probably earlier.

lokum

English loo for toilet is probably from the French lieu. The locum tenes mentioned in Garg's etymology has become the lieutenant of today in English.

I love the Danish word for birthday ; Fuzzeldei (if I remember well) I understand 'dei' must be related to day, but can you tell me if you please what's the fuzzel? Welcome.

Pardon my ignorance, but how does the person who "holds the
place" for someone do that? What is the job? And for whom?
I've often been confused by the term.
Also where does the British "leftenant" come from? "Lef"?

how does the person who "holds the place" for someone do that? What is the job? And for whom? I've often been confused by the term.

Taking somebody's place? When is a placeholder a lieutenant? How did Latin officium 'service, kindness, favor; duty' come to mean a room in which work gets done? Though we do have officeholders, too. Latin locus had a plethora of meanings: 'place, spot; seat, lodgings; (in war) post, station'. (See the entry in Lewis & Short.)

Also where does the British "leftenant" come from? "Lef"?

It's a complicated story, but basically, the pronunciation with f is older. See the entry in the OED1:

THANKS, GRATEFULLY.
It is amazing how just using a word like "office-holder" can
clear up the clould of confusion.
Lef is older, and seeing the quote, I now understand it, and
I appreciate your taking to time to help me out. Merci.