Snigger

"Stop sniggering!" my mother always directed.  So what did she mean?

To snigger is to laugh quietly and, most importantly, disrespectfully.  The word is also the a noun.

And where did it come from?  The dictionary suggests that it's an alteration of another word for the same thing--snicker.  And that word?  "Perhaps imitative"says the dictionary.  But to me, that sounds like a stretch....