Viscous is an adjective that means having a thick, sticky consistency.
The Latin word viscum (VEES coom) refers to birdlime, a sticky substance used on twigs to trap small birds. Something that is described as viscous may not have anything to do with birds or birdlime, but it has in common that sticky consistency.
As a bartender, I discovered, after trying to experiment with my own concoctions, that people prefer drinks that go down nice and smooth. Anything too viscous is not likely to gain many fans.