Viscous means sticky, gluey and syrupy. So if something is viscous, you usually don't want to stick your fingers in it — that goes for boogers and maple syrup alike.
Not quite a solid and not quite a liquid, scientists like to say that viscous things don't flow very easily. They glop and slug around slowly, sticking to whatever they come in contact with. Think of making a batch of Rice Krispie treats: One minute the marshmallows are solid little rounds; the next minute they're melted into a web of sticky white goo that's somehow attached itself to the Rice Krispies, the spoon, the counter tops, your nose, the front door, and the dog.
The rules' reach may even extend to oil produced in California, where steam injection is used to extract viscous crude from old fields.