I have a bit of trouble understanding what a smirk really is. Is it always vicious or is just a small, ambigiuous, not necessarily vicious, smile?
A dictionary might help. smirk: Smile in an irritatingly smug, conceited, or silly way. (Oxford Dictionary) In a smirk one side of the mouth is usually turned up a little more than the other side.
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My dictionary defines a smirk as a conceited,smug, scornful or silly smile.
Here is a definition of 'smirk' as a verb, for learners of English, from Merriam-Webster.
Learner's definition of SMIRK[no object] : to smile in an unpleasant way because you are pleased with yourself, glad about someone else's trouble, etc.