Have you already filled out the customs form? If not, a half-truth like "Vegetable Glycerine" would be okay. I've seen "flavorings" and "scented oils" too.
"Nicotine" or "ecig liquid" will likely be denied entry, assuming they read it.
Oh, and it's not illegal to send us nicotine in the mail. Don't worry. It's only banned from importation so it will be returned to you, if they deny it entry.
Mail fraud, however, is another subject and it can get tricky. Personally I think it's pretty damn rare to be charged over an incorrect declaration form, unless it's for something serious, like illegal weapons. Moreover, no half-truth like "Glycerine" would be indicative of intent to defraud.