気 means attention or mind, を marks the object, and つけて is the connecting form of つける. The fixed send-off asks the listener to stay alert.
気 means attention or mind, を marks the object, and つけて is the connecting form of つける. The fixed send-off asks the listener to stay alert.
When is it said?
As someone leaves or sets out.
Is it only about a specific danger?
No. It can be a general wish for a safe journey.
Is it polite?
It is familiar; 気をつけてください is more formal.
Can it answer 行ってきます?
Yes. It is a natural send-off.
Does it mean get well soon?
No. For an ill person, お大事に is more natural.