1.
like cats and dogs; on very bad terms; bitter rivals
An idiomatic expression describing two people or groups who are always fighting or who cannot get along at all. Literally means "the relationship of dogs and monkeys," as dogs and monkeys are traditionally believed to be natural enemies in Japanese culture.
あの二人は犬猿の仲だ。
Those two are like cats and dogs.
隣の部署とは犬猿の仲で、会議ではいつも対立する。
We're on terrible terms with the neighboring department and always clash in meetings.
子供の頃は犬猿の仲だった兄弟が、大人になってからは仲良くなった。
The siblings who were like cats and dogs as children became close after growing up.
One of the most well-known Japanese idioms. The expression comes from the folk belief that dogs (犬) and monkeys (猿) are natural enemies. This motif also appears in the folk tale 桃太郎, where a dog and a monkey reluctantly cooperate. Note that Japanese uses "dogs and monkeys" where English uses "cats and dogs." Always used in the pattern ～は犬猿の仲だ. Related: 水と油 (oil and water — also describes incompatibility), 仲が悪い (on bad terms — straightforward expression).