Vicious means (1) evil, immoral, or depraved; (2) spiteful, malicious, (3) aggressive, and (4) severe or intense. Viscous means having relatively high resistance to flow (think mud or industrial sludge). The two are sometimes carelessly mixed up.
Examples
Here are a few examples of the adjective viscous (whose corresponding noun is viscosity) used well:
Gallons of the viscous amber fluid have been given to politicians and journalists, as the apiarists of Australia lobby for action against the imminent incursion of Asian bees. [Sydney Morning Herald]
The thick and heavy bitumen, sometimes called tar or tar sands, is too viscous for traditional pipeline transportation. [The Cherokeean Herald]
Once there, it will reduce the viscosity of the thick crude oil residing in the well and allow for easy extraction. [Inhabitat]
And vicious (whose noun is viciousness) is used well in these sentences:
Minks are vicious carnivores. [NPR]
An ordinance providing for the regulation of vicious and potentially dangerous dogs was introduced on first reading by the township committee this week. [NJ.com]
I defend their right to be vicious and I don’t take it personally anymore, because I see literary viciousness as a dark art that sometimes needs writers as its canvas. [The Guardian]
