Of the various meanings which Juha has given, I'd recommend you only use the word 'hectic' in the first sense, meaning 'busy' or 'fast-paced'. For example: 'I had a hectic day yesterday' 'Things are really hectic at work at the moment' 'If you don't enjoy a hectic lifestyle, don't go to live in a big city.' Note that a time, a place, a life or an atmosphere is described as hectic - not usually a person. I've never heard the second or third usages which Juha gives (dangerous/crazy), so I'll presume he's right in saying that they must be Australian slang.

Hectic has a few meanings, firstly it means when something is very busy to somebody. For example ; I had a hectic afternoon = he had a busy afternoon! It can also mean, when something is very dangerous to somebody. Example ; This is too hectic = this is too dangerous or borderline dangerous. It can also mean that a person is a little crazy ; he's abit hectic = he's abit crazy or wild! This is in Australian slang, I am not sure if hectic is used anywhere else in the English world.