Godan verb with 'mu' ending, Transitive verb
to take for granted; to assume; to be convinced (that); to jump to the conclusion (that)
本人は唯我独尊を決め込んでいるようだけども、周りから見れば単なるわがままだよね。He's completely convinced of his own greatness but to the people around him he's just a selfish jerk. 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
Also written 決めこむ【きめこむ】、決込む【きめこむ】、極め込む【きめこむ】