Fetching details from the dictionary...
to take for granted; to assume; to be convinced (that); to jump to the conclusion (that)
to pretend (to be); to pose (as); to fancy oneself (to be); to feign (e.g. ignorance)
to go through with (what one decided to do); to persist in doing; to make up one's mind
彼は自分が正しいと決め込んだ。
He took it for granted that he was right.
彼女は誘いを断られると決め込んでいる。
She is convinced that the invitation will be rejected.
事実確認をせずに答えを決め込む癖がある。
There is a tendency to assume answers without checking the facts.