As Chris says, 'mean' used in this way means cruel. It can also mean malicious, unfair or 'nasty'.
This is an American usage which has slowly become international. I grew up in a language environment where the only everyday meaning of 'mean' was tight-fisted: the opposite of generous. Nowadays, young people everywhere are influenced by American media and use 'mean' to refer to unkind behaviour towards someone. Language changes.
Now here's a question. How many italki members know what 'captious' means? I'll admit that I didn't until 5 minutes ago when I looked it up in a dictionary. It means 'very critical', I discover.
Tip: if you google a word and the only results you get are dictionaries and thesauruses, you know that it isn't particularly useful or common. But thank you, Sasha, for teaching me an English word!