round peg in a square hole; mismatch; incompatibility
A rare yojijukugo meaning that things do not fit together, like a round peg in a square hole. Used metaphorically for incompatible ideas, people, or situations.
円鑿方枘 is a classical idiom that expresses incompatibility, like trying to fit a round tenon into a square hole.
A Sino-Japanese idiom from classical Chinese 円鑿方枘, “round mortise and square tenon,” an image for two things that cannot fit together.