Weasel words are words or phrases used in an ambiguous manner in order to make a point. This can also apply to selling a product or spreading a meme. These words often do not reflect the whole truth about the point, product or meme they describe - indeed, they can often be used to actively mask the truth. As such, weasel words may be used to deceive or avoid having to back up assertions.
It has been suggested that the phrase comes from an old folk-belief that weasels sucked the contents from birds' eggs to leave only an empty shell.
Probably the most common form of weasel words are those that make a topic appear accepted without providing anything specific to back it up - the validity of the point then rests on the ambiguity provided by the weasel term. For example, a phrase might be that "Volkswagens are widely considered to be rigid". This assertion may or may not be true, but by itself it tells us absolutely nothing about who considers it to be true, why they think it's true, nor how wide "widely" is.
Weasel words are sometimes a form of argument from authority or argument from popular opinion, which many people believe are logical fallacies. Weasel words are widely considered to be a sign of intellectual laziness. Most people use weasel words a lot without realizing it, even when they are writing an article on weasel words. This problem is believed by many to be widespread (see what we did there?)