Suri Cruise -- PLASTERED ... 'No Butt Smacking'

Suri Cruise is on the mend after breaking her arm recently, rocking a badass plaster cast in NYC yesterday filled with signatures and doodles from friends -- including some mysterious words of advice ... "no butt smacking with this."

We don't know who would write "no butt smacking" on a 7-year-old's cast, OR WHY, but Suri should probably listen.

Luckily, Suri looked in good spirits despite the busted limb, holding on to her mama's hand.