This is the 13445th most frequent Swedish word.
upptagen
"Upptagen" means "busy" or "occupied" in English.
In this context, 'upptagen' is used to describe someone being mentally absorbed or preoccupied with something.
Han såg upptagen ut med sina tankar.
He looked preoccupied with his thoughts.
In this sentence, 'upptagen' is used to describe being occupied or busy with a task or activity.
Jag är upptagen med jobbet hela dagen.
I am busy with work all day.
Here, 'upptagna' describes something being occupied or used, in this case, the train being full of people.