Lighten up, cmh and Jacki! Next you'll be complaining about shortening
telephone to
phone.
I don't recall seeing or hearing
sesh, but I see it is in use here in Australia. For example the
Chronicle, of Toowoomba in Queensland, on 22 September:
I've put a table and some chairs in my back yard when I've had a few friends over for a Sunday afternoon sesh and we've had a good afternoon but I never claimed it was a beer garden.
(The OED identifies the colloquial sense of
session, meaning a "period devoted to heavy drinking; a drinking-bout", as chiefly Australian.)
And as you say, MK, it's not unknown in the US these days. The New York-based online publication
Elite Daily ("the ultimate digital destination for millennial women who are discovering the world, and themselves in the process"), on 28 October:
And while you can apply it for a quick 10-to-15-minute masking sesh, you can also feel free to leave it on overnight in lieu of your go-to moisturizer, as it doesn't necessarily need to be washed off.
In future
sesh and its plural
seshes will be accepted as rare words.