We recommend that you sit up and take note of this article by Tim Bowen. You might learn something new!
The most widely-used phrasal verbs formed with sit are sit down and sit up, where the meaning is broadly literal. Used with other particles, however, most phrasal verbs formed with sit have an idiomatic meaning.
If you sit through something, for example, you have to stay until the end of it and it is probably something that you do not enjoy, as in ‘If I have to sit through one more boring meeting, I think I’ll scream’.
If you sit something out, you stop taking part in something such as a game or a dance for a short time, as in ‘I’m tired so I think I’ll sit out the next dance’.
To sit around or sit round means to spend time in a lazy way, doing nothing important, as in ‘Since Dad lost his job he just sits around the house all day’.
If you sit in for someone, you take their place temporarily, as in ‘I’ll be sitting in for the secretary at the meeting tonight’ (stand in for can be used in the same way), whereas if you sit in on something, you go to a meeting or a class although you are not directly involved in it, as in ‘Do you mind if I sit in on your class this afternoon’.
To sit by means to make no effort to stop something bad from happening, as in ‘Are we just going to sit by and watch them take over the company?’
Finally, to return to sit up, this verb can also mean to suddenly notice something and realize that it is important or serious, as in ‘If these figures don’t make them sit up and listen, nothing will!’.