A schlemiel is a somewhat dopey, awkward person. You might angrily call your brother a schlemiel after he drops the cake you've spent all day making, stumbles, and sits on it.
Nothing every goes quite right for a schlemiel — he's the guy who never seems to get any good luck, and who's less than brilliant and not particularly graceful. A schlemiel in a movie is an object of fun, perfect for laughing at. In real life, it's pretty hard to be a schlemiel. The word schlemiel means "bungler" in Yiddish, from a character called Peter Schlemihl in a classic German fable.