A shlemiels is a person who is always unlucky or who often makes mistakes.
Calling someone a shlemiels is not very nice.
Examples:
He is such a shlemiels, he always drops his ice cream.
Don't be a shlemiels and spill the milk.
The shlemiels forgot his umbrella and got wet in the rain.
The poor shlemiels tripped over his own feet.
He's the biggest shlemiels I know, always messing things up.
The shlemiels broke the window with the ball.