Ambiguous or quibbling speech.
It has long been a widespread belief that weasels suck the yolks from bird's eggs, leaving only the empty shell. This belief is the basis of the term 'weasel words', used to describe statements that have had the life sucked out of them. The expression refers to words that are added to make a statement sound more legitimate and impressive but which are in fact unsubstantiated and meaningless. Examples of weasel words are 'people say that...', 'studies show that...', 'up to 50% or more...'.
There is now some doubt amongst naturalists as to whether weasels do suck eggs. The tiny mammals are certainly ferocious and, pound for pound, amongst the most dangerous predators on the planet, being easily able to kill an entire coopful of chickens that are hundreds of times their weight. They have a bad reputation with country dwellers but the egg-sucking behaviour is unproven. Whether or not they actually suck eggs, Shakespeare and his contemporaries believed they did. The Bard didn't coin the expression 'weasel words', but he came very close, when he made two references to the supposed habits of weasels:
The weazel Scot Comes sneaking, and so sucks the princoly egg. - Henry V, 1598
I can suck melancholy out of a song, as a weazel sucks eggs. - As You Like It, 1600
That's as close as we get to the actual phrase in the Tudor period and it wasn't until the turn of the 20th century in the USA that the phrase 'weasel words' first occurred in print. In 1900, Stewart Chaplin published a story in The Century Illustrated Magazine titled Stained Glass Political Platform, which contains this exchange:
"I am the chairman of your committee on platform"... "And like most platforms," continued St. John, "it contains plenty of what I call weasel words."
"And what may weasel words be?"
"Why, weasel words are words that suck all the life out of the words next to them, just as a weasel sucks an egg and leaves the shell."
In 1910, Theodore Roosevelt, then Colonel Roosevelt, was reported in various US newspapers as saying that he liked the Republican state platform because it contained no "weasel words". In September 1916, the New York Times published a piece in which Roosevelt refuted the notion that he had stolen the phrase from Chaplin and claimed to have coined it independently in 1879:
Colonel Roosevelt, on his way here this morning from Portland, Me., told a Times reporter how he happened to use the expression "weasel words" in describing some of President Wilson's utterances months ago. After the expression had been widely quoted, somebody discovered that it had been used years ago by the writer of a magazine article in the Century Magazine, and the Colonel was charged with having plagiarized the writer.
"About thirty-seven years ago." Colonel Roosevelt said in talking of the origin of the expression. "I was going up a mountain in the Maine woods in a carriage, driven by Dave Sewall. We saw an old man along the roadside. When we passed Dave Sewall said:
"That there man can do a lot of funny things with this language of ours. He can take a word and weasel it around and suck the meat out of it like a weasel sucks the meat out of an egg, until it don't mean anything at all. The Colonel said the expression [weasel words] occurred to him when he read some of President Wilson's notes.
It is possible that [there are some good weasel words for you] Roosevelt coined the expression but, of course, his later recollections aren't any kind of proof of that. If circumstantial evidence counts for anything then Roosevelt's etymological track record might be called into account. In 1900, he described the phrase 'speak softly and carry a big stick' as a 'West African proverb'. Where he got that idea from is unclear - there's certainly no evidence to support it.
I can't finish without adding the old jest about how to tell a weasel from a stoat - 'one is weasily recognized, the other is stoatally different'.