"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".