What does exactly mean ‘to plant oneself somewhere’?
On MacMillan dictionary we read:
to firmly put someone or something or yourself in a particular place or position.
Plant something in/on etc something:
"Henry planted himself in the seat next to me."
But what emotion does this action convey specifically? What could be the reason for Henry to plant—rather than just seat—himself in the seat? Does the word, in this case, suggest that Henry was tired, angry, or that he wanted to make sure that no one else would steal that seat?
To me, "Henry planted himself in the seat" carries the connotation that he is firmly sat and has no intention of giving up his seat any time soon. Depending on the context, it may be inferred by extension that Henry is not welcome.
Here are some dictionary examples:
She was determined to keep both feet firmly planted on dry land. (Oxford)
She planted her feet firmly to the spot and refused to move. (Longman)