bee in one's bonnet

a bee in one's bonnet — a fixed idea/an obsession that seems crazy;

have a bee in one's bonnet — be obsessed with something; to be very involved in something that one thinks is important, in a way that other people find annoying — зациклиться, застрять на чем-то; иметь пунктик по какому-л. вопросу; иметь навязчивую идею;
(bonnet = hat)

Example 1: She's got a bee in her bonnet about eating raw fish to stay fit and healthy. — У неё пунктик / она зациклена на том, что необходимо есть сырую рыбу, чтобы сохранить здоровье.

Example 2: Bert didn't get the promotion he wanted last year, and he's had a bee in his bonnet about it ever since. — С тех пор как Берту не удалось получить в прошлом году долгожданного повышения, он закциклился на этом/ это не дает ему покоя.

An alliterative refinement of an earlier expression 'his head is full of bees' (~ с тараканами), i.e. he is scatterbrained, unable to think straight, as if he has bees buzzing around inside his head. The notion of having a bee in one's bonnet implies an inability to concentrate on anything else.