People say the funniest things when they think no one's listening, or can't understand.
I was checking out of my hotel in Mexico, and the young lady behind the desk was joking around with one of her co-workers.
I didn't catch what he said, but she jokingly replied:
Of course I had to laugh.
If you don't know, "callate", it's along the lines of "shut up". I wouldn't say it's that strong, but it's up there.
"callate la boca" would definitely qualify as being rude. You can also say "cierra la boca" which is quite possibly equally strong. Personally, I've avoided using "callarse", because I'm still unsure of just exactly how strong this is. And of course, your tone of voice plays a huge part of how things are inteprepted.
But if you want to ask someone politely to be quiet, you can't go wrong with "Silencio, por favor".
But let's move on to the more interesting word, "chismolero".
I hadn't heard this word before, but I knew that a "chismoso" is a person who likes to gossip, so it wasn't much of a stretch to figure out that a "chismolero" is a synonym.
The verbs chismear and chismorrear mean to gossip, so now you've got all the tools you need to start discussing all the going-ons in your neighborhood with your Spanish neighbors.