So, a quick Google search of "dewar" tells me that someone famous is named Dewar, along with Dewar's Whiskey, and there is a certain type of flask called a "Dewar flask" (capitalized).

I think I should still be able to tell Word that I don't want it capitalized, though, right?

For reference, it is not common in non-chemistry communities to capitalize "dewar." here is a NASA web page that has it in lower case (see the caption under the picture):

http://www.nasa.gov/mission_pages/gpb/gpb_008.html