Weasel words: words that seem to promise more than they deliver. Weasels can allegedly suck the contents out of an egg without breaking its shell; analogously those who use weasel words suck the meaning out of a sentence while apparently leaving it intact. This term is not particularly precise. The range of cases it covers is best illustrated with examples. There are at least two main uses of the term.
First there are some weasel words, so-called, which are simply imprecise. So, an advertiser who declares that the food they are selling is a ‘healthier alternative’ needs to specify precisely what the food is healthier than and why for this to mean anything at all. Otherwise ‘healthier alternative’ is a kind of rhetoric, a weasel phrase.
A distinct use of the term ‘weasel words’ refers to equivocation, such as in the ‘No True Scotsman Move’. So within an argument, someone might say ‘All truly intelligent people have a sense of humour’ and the when confronted with someone with an extraordinarily high IQ but no sense of humour, declare that this person is not a counterexample but rather ‘not a truly intelligent person’. Here ‘intelligent’ is the weasel word, the one that is re-defined in the course of the discussion. (This second sense is the one given by Anthony Weston in his excellent short book A Rulebook for Arguments, 3rd edition, p.78).