File this one away for future reference. If someone looks drossy, then they look so unpleasant that you presume they must have a terrible attitude as well.
That—like the lion’s share of words-you-didn’t-know-you-needed—is a Scots dialect word, with the definition we posted on Twitter being taken from John Jamieson’s monumental Etymological Dictionary of the Scottish Language (1825).
But speaking of etymology, where exactly does drossy come from? And what, for that matter, is dross?
First of all, you’d be right for thinking the two words here are related. If you look drossy, then put simply you look like dross—a word that has been used to mean “dregs” or “refuse” for nigh on a millennium, since the late Old English period at least.
Although we use it generally of any waste material today, originally dross was a slightly more specific term, used to refer to the extraneous scum ejected or left over from the process of smelting metals. But by the fifteenth century, that meaning had loosened so that dross had come to be used of any waste that, due to its being mixed into a finer or more valuable material, renders it useless of impure. So the likes of the silt that settles in muddied water, or the sludgy lees left in a vat after fermentation, would all be known as dross.
That looser meaning (an example of a process called semantic widening, or generalization if you want to get all linguistic about it) continued to develop apace over the centuries that followed, so that by the turn of the sixteenth century dross had all but fully established itself as a general term for any waste matter. A handful of more specific meanings of dross remained in use for a time (most notably, as another name for iron pyrites or so-called “fool’s gold”), but largely speaking it’s the more general meaning that has survived—and, moreover, inspired the brilliant adjective at the top of this page.