It originated on the Chapo Trap House podcast (e.g. in the thumbnail of this video). Not sure when the earliest use of it was, but it was at least early 2019 from a quick search through Will’s twitter. It refers to how Trump’s photos show, or at least used to show, him as slightly moist, damp, or sweaty. A slight sheen of water seemed to be on him at all times, like some fucked up amphibian. Compare this to the eternal dryness of other politicians like Biden.