It means 'happen' more in the sense of be successful or manage to happen in spite of difficulties. "The meeting is not going to come off" This makes sense. It means, for example, that you realise you'll never be able to get the right people in the same place at the same time. "How did your speech come off?" This is also possible. You are asking if the speech was successful or well received. "The party is coming off in the open air at 5 pm". No. This one makes no sense. It isn't a synonym of 'take place' or 'be held'. There needs to be an element of risk involved. The 'off' idea suggests an achievement of some kind against the odds. This phrasal verb is related to others such as 'bring off' and 'pull off', which we use for tricks, stunts, pranks, frauds and deceptions. The idea is that someone succeeded, through skill and ingenuity, in making these things happen.

On further thought, we would usually say "come off it" rather than "come off that"

I agree with all Su.Ki's comments above. Would like to add there's also an idiomatic expression. The interjection "come off that!" means "stop that!", where the speaker thinks the person they're speaking to is doing or saying something silly or otherwise undesirable.