勝手に, how to use it?


My Japanese teacher wrote this as an example :

> **勝手【かって】に** 人の部屋に[上]{あが}り[込]{こ}むなんて、失礼な人だ！  
>  It is a rude person to come into someone else room without permission.

I figure out that 勝手に meaning was : **without permission**. But the
translation on internet is quite different, it has the sense of "voluntarily"
which is not the same thing to me. I was just wondering if this was a mistake
of my teacher or if this has actually 2 meanings.




The basic meaning of 勝手に is "of one's own accord", ie. doing something on
one's own judgement without consulting anyone else.

When used of a person, it usually (but not necessarily always) has negative
connotations, implying that the person isn't being very considerate and they
_should_ really have consulted someone before doing the thing. This negative
connotation is particularly explicit in the adjectival equivalent 勝手な(人),
which refers to the kind of person who only thinks of themself.

勝手に can also be used with non-human subjects (機械が勝手に動き出した "the machine started
moving on its own", 問題が勝手に解決した "the problem resolved itself") in which case
there is of course no value judgement involved.

I have trouble thinking of many situations where it would be natural to
translate 勝手に as "voluntarily", but it's not out of the question, since they
do both indicate that a person is doing something of their own will rather
than due to anyone else's influence.

