Tim Bowen knocks us out with his explanation of this entertaining phrasal verb.
On the face of it knock does not appear to be a very interesting verb. Its most common meanings are associated with hitting things, normally doors ('They walked up to the door and knocked loudly') or people ('The force was powerful enough to knock him to the ground'). There are, however, some quite entertaining phrasal verbs based on knock, including one or two examples which require considerable care to be exercised. One such verb is knock off, which as a transitive verb can mean to reduce the price or amount of something, as in ‘Kelly knocked two seconds off her previous time', or ‘The supermarket is knocking 50p off a bottle of wine’. As an intransitive verb it can mean to finish work for the day, as in 'Do you want to knock off early tonight?' To knock off is also a slang expression that means to kill, as in, 'the assassin knocked off the government minister.'
Unfortunately, in British English at any rate, to knock off can also mean to have sex with someone and, to make matters worse, if a man knocks a woman up, it means (particularly in American English) that he makes her pregnant. This can lead to great intercultural amusement because in northern British English to knock up is more commonly used to mean to wake someone up or call round on them by knocking on their door, as in ‘Can you knock Helen up tomorrow morning?’ Knock up can also mean to produce something quickly and easily, e.g. ‘It doesn’t take long to knock up some pasta’.
If all of this seems too fraught with danger to risk using, you might like to knock around (spend time) with your friends for a while, perhaps knock back a few beers (drink them quickly) and knock them out or knock the spots off them (impress them very much) with your knowledge of phrasal verbs. However, if you go on too long, your friends may tell you to knock it off (stop) or they may get annoyed and knock your block off (punch you in the head - usually used in a jovial way).