Yas Yas....was the jazz/blues slang term for arse.
That's Your Yas Yas
Keep Your Yas Yas Clean
The Duck's Yas Yas
Get Your Yas Yas Out
The Yas Yas Man
The Yas Yas Girls
Memphis Minnie - New Dirty Dozen
Now the funniest thing I ever seen, tom cat Jumping on the sewing machine
Sewing machine running so fast took ninty-nine stiches in his yas yas yas
As for the case of the Stones 'Get You Ya-Ya's Out!'
It's most possibly a nod to the blues "arse" term....
As in get your arse out and shake it ...BUT there could also
be a chance it might be meant as also getting your verbal cheering and
yelling out for the Stones as in.....Get Ya-Ya's (Yeah - Yeah's) Out...
ROCKMAN