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 countertops, your nose, the front door, and the dog.