selfishly; without permission; on one's own accord
Describes acting on one's own judgment or desire, often disregarding others' wishes or rules. Can be neutral ('voluntarily') or negative ('selfishly').
He entered my room without asking.
The door opened by itself.
From 勝手 (katte, 'one's own convenience' or 'kitchen') + adverbial particle に. The sense of 'selfishly' developed from the idea of doing things according to one's own convenience.