For advice on the use of weasel words in editing Wikipedia, see Wikipedia:Avoid weasel words.
A weasel word is a word that is intended to, or has the effect of, softening the force of a potentially loaded or otherwise controversial statement. This phrase appears in Stewart Chaplin's short story Stained Glass Political Platform published in 1900 in The Century Magazine according to The Macmillan Dictionary of Contemporary Phrase and Fable : "Why, weasel words are words that suck the life out of the words next to them, just as a weasel sucks the egg and leaves the shell." Thus, weasel words suck the meaning out of a statement while seeming to keep the idea intact, and are particularly associated with political pronouncements. Weasel words are used euphemistically. The term invokes the image of a weasel being sneaky and well able to wiggle out of a tight spot. Weasel words work, ad nauseam, as in commercial lingo to glide over an uncomfortable fact (therefore "headcount reduction" replaces "firing staff") encouraging people to identify and nominate examples of weasely language, which gives many examples of dissimulation through verbosity. Watson was previously a speech writer for Australian Prime Minister Paul Keating.
"I realized that the purpose of writing is to inflate weak ideas, obscure poor reasoning, and inhibit clarity." Bill Watterson (1994) Homicidal Psycho Jungle Cat page 62

Purposes
In certain kinds of advertisements, for example, the part of the syntax that would normally establish the validity of a statement is missing or is being withheld deliberately in the expectation that the listener or reader will complete the message subliminally and so will be influenced by it:

"... is now 20% cheaper" (It is now 20% cheaper than what?)
"There is more goodness in ..." (How is this goodness measured and of what does it consist?)
"More people than ever are using ..." (What does that mean in numbers?)
"New and improved ..." (Improved in which qualities? If it is "improved", how can it also be "new"?)
"Our ... will never be cheaper." (Is this accounting for inflation? Is your profit margin thin enough that you could not have a cheaper sale next year?)
"Clinically tested..." (but not proven? What did the test results reveal? Does the product work as claimed?)
"Four of every five people would agree." (Is this a good sample population? Were only five people interviewed?)
"... is among the" or "... one of the (top, leading, best, few, worst, etc.)" (How many else among? What percentage are not among? Where does the one rank among?) Use of generalizations

Essentially contested concept
Spin (public relations)
Political correctness
Glittering generality
Newspeak
Sapir-Whorf hypothesis