Vicious and Viscous are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect also tries to anticipate what word you want based on the first few letters. Words sharing letters doesn’t equate to words sharing a meaning. Autocorrect’s suggestions are often more funny than accurate.
Vicious is an adjective. It describes angry, ruthless behavior by a person or animal. When a dog has been trained to attack or fight to the death, it’s described as ‘vicious.’ Wild animals that will kill anything for food or defense are also described as vicious: bears, snakes, big cats, spiders.
Viscous is an adjective. It means sticky, or having an adhesive texture that adheres to surfaces and clothes. Its used in chemistry and scientific writing more than day to day language.
The following sentence uses both words correctly:
Victor was a chemist who played bass in a Sex Pistols cover band. As a play on “Sid Vicious”, his stage name was “Vic Viscous.”