"Steady My Head"
To be
said after something
ridiculous or embarassing (to someone else) happens.
Essentially telling
someone that you need to have your head "steadied", in reference to the head shaking in dissaproval or moving with laughter, because what has just occured it so outrageously funny or embarassing to the other person.
This can be pronounced verbally as "smhhh" or "smuh".