I think most of us are familiar with the common semantic stopsigns like "God", "just because", and "it's a tradition." However, I've recently been noticing more interesting ones that I haven't really seen discussed on LW. (Or it's also likely that I missed those discussion.)
The first one is "humans are stupid." I notice this one very often, in particular in LW and other rationalist communities. The obvious problem here is that humans are not that stupid. Often what might seem like sheer stupidity was caused by a rather reasonable chain of actions and events. And even if a person or a group of people is being stupid, it's very interesting to chase down the cause. That's how you end up discovering biases from scratch or finding a great opportunity.
The second semantic stopsign is "should." Hat tip to Michael Vassar for bringing this one up. If you and I have a discussing about how I eat too much chocolate, and I say, "You are right, I should eat less chocolate," the conversation will basically end there. But 99 times out of a 100 nothing will actually come out of it. I try to taboo the word "should" from my vocabulary, so instead I will say something like, "You are right, I will not purchase any chocolate this month." This is a concrete actionable statement.
What other semantic stopsigns have you noticed in yourself and others?
Because professional mathematicians understand and depend on the technical usage, there's little risk of the technical sense becoming diluted by such quasi-humorous, figurative allusions to the technical jargon, which can serve as a means of in-group bonding. When outsiders do it, hover, it's no longer clearly an allusion to something else, and risks being mistaken for a distinct technical usage in its own right, in addition to losing the slight humor/bonding value.
Another mathematical in-term that has been subject to similar abuse by outsiders is the word "isomorphic". When a mathematician speaks to a colleague of all the local cafeterias being isomorphic, this is clearly hyperbole -- but it's only clear if one understands the actual meaning and normal context of the word.
From what I've seen of cafeterias on large college campuses, it isn't actually hyperbole to say that "all the local cafeterias are isomorphic". They're technically distinct, but under a transformation that preserves all relevant properties, they can all be mapped to each other; they are the same up to isomorphism.