平気

1. Origins, structure, and basic meaning

• Structure: 平気, read へいき, is a na-adjectival stem meaning all right or unconcerned.
• Basic usage: 濡れても平気です says the situation is acceptable; 平気な顔 describes an unconcerned expression.

2. Common pitfalls and similar expressions

• Predicative 平気です and attributive 平気な
  • The polite predicate evaluates a situation, while な modifies a person's expression; English adjective choice follows the subject.