And according to the Alt Usage English:
The most plausible explanation is the one given in the latest edition of Collins English Dictionary: an alteration of "Mind your 'please's and 'thank you's".
They also had other possibilities listed.
"Mind your 'please's and 'thank you's".
ThatI have heard before!
Reminds me of a joke:
Q: Why don't Junior Leaguers like group s-x?
A: Too many T-notes, darling.
(Am I showing my age.....?)