What does סתם mean?
Like JessikaMorgan said, it is nearly impossible to determine w/o context, due to the "marvelous" feature of Hebrew. So it could mean either "stopper" or "plug" or even "tampon". It could also be a third single pronoun verb, meaning "stopped" or "blocked". And then it could mean something like "just like that ..." (without a special reason). For example: למה זרקת את הכדור? סתם... (Why did you throw the ball? Just like that ... no special reason)
I believe here its mean "for no reason" even thuough its not a word you hear often.
I(and others) use it 99% of the time as "Just Kidding", but it is slang
Stam could means "for nothing" as in "Why did you do that? Stam". It could also mean "just joking". It's just a slang word.