expression, godan verb (-ru)
be self-satisfied; chuckle to oneself
Describes someone who is quietly pleased with their own actions, ideas, or situation, often with a sense of smugness or private delight. The phrase is a set expression and not broken down further.
彼は自分の発明に一人悦に入っている。
He is pleased with himself over his invention.
会議で意見が
After his opinion was accepted at the meeting, the department head chuckled to himself with satisfaction.