proper, fitting, one's own
If you expropriate something, you take it away for your own use although it does not belong to you; governments frequently expropriate private land to use for public purposes.
When something is apropos, it is fitting to the moment or occasion.
If you appropriate something that does not belong to you, you take it for yourself without the right to do so.
A malapropism is an unintentional and usually humorous mistake you make when you use a word that sounds similar to the word you actually intended to use but means something completely different.
Propriety is behaving in a socially acceptable and appropriate way.
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