Weasel words are words inserted into a claim that make it weaker than it initially appears. The graphic below gives a number of examples of a favorite weasel words of advertisers: the phrase
"up to." "Up to" is a brilliant weasel word because it can make it sound like there is an incredible sale and one could get a great deal. The first image promises "Mega Reductions" and prominently features "70% off" making it sound like one could save almost 3/4. However, that weaselly "up to" actually reveals that not everything will be 70% off, and that 70% is the absolute most that you could possibly save. Such a claim is compatible with one small item being 70% off and everything else being normal price. If you tried to complain that the store was guilty of false advertising, the owner could easily respond by noting that they didn't promise that everything was 70% off, merely that one could save 'up to' that amount. The same sort of issue applies to all the examples presented below.
h/t to Angust at cracked.com