A shamus is a guy who takes care of handyman tasks around the temple, and makes sure everything is in working order. A shamus is at the bottom of the pecking order of synagog functionaries, and theres a joke about that:

A rabbi, to show his humility before God, cries out in the middle of a service, Oh, Lord, I am nobody!.

The cantor, not to be bested, also cries out, Oh, Lord, I am nobody!

The shamus, deeply moved, follows suit and cries, Oh, Lord, I am nobody!

The rabbi turns to the cantor and says, Now look who thinks hes nobody!