For English speakers it's a really not subtle hint that he's evil.  It's like naming a villain "Bad Man".  The name Sidious by itself isn't a word, but it's probably derived from the word insidious.  [Google](http://www.google.com/search?q=define+insidious) defines the word insidious to be `Proceeding in a gradual, subtle way, but with harmful effects` which is a good description of what Darth Sidious/Palpatine did in the prequel trilogy.