What is the meaning of "I'm a wiener"?

I have scoured out dictionaries searching for the word wiener without success. As you can notice I am not a native speaker. I usually see the this phrase as a demeaning joke on animations when some character is lured to say one's name when actually it turns out as something stupid like "I'm a wiener" or "I. C. Wiener".

Can someone say where is the joke on it? Because it sounds to me like "winner", what is not a demeaning word, reather it comes as a comendation.

Solution 1:

I. C. Wiener = icy wiener, the latter literally "sausage" but slang for penis. (Of course "penis" itself is quite the euphemism treadmill over the years, with hundreds if not thousands of terms; even the word itself comes from a Latin euphemism, literally "tail".)

It's juvenile rather than demeaning as such. Note also the connection with "dick", which means both "penis" and "jerk, contemptible person".