(literally a "piece of shit"); when referred to people = insult referred to someone who's a midway between a asshole and a bastard, sometimes both ... depending on the construction of the phrase.
sei proprio stronzo = you are a "bad fellow" (kidding someone jokingly)
quello è proprio uno stronzo = that bloke is really a hopeless bastard come si fa ad essere così stronzi? = how can anyone be such an asshole?
Stronk is an eastern European/Russian pronunciation of the word "strong." Most commonly used in the context of mocking or showing reverence towards something brutish or powerful that is of eastern European/Russian origin.
A typo of the word "strong," originating from competitive play of a game of Heroes of Newerth. Mostly used sarcastically because of it being a mistype.