思い込む

おもいこむ · omoikomu · JLPT N2 · verb-godan

to be convinced that

思い (thought) + 込む (pack in, go deep) = a thought packed in so tight it is stuck fast

込む means to crowd into or sink deep, so 思い込む is a thought that has sunk in so far it feels settled as fact, even when it may be wrong.