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 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.
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