You say it when someone starts doing the same (usually unpleasant or annoying) thing again.

When my cat bitted my flowers, I said: "Don't do this!"..and he runned under table.. But when I went to the kitchen..I heard..he jumped to flowers again..I said: "Ты опять за своё?" I ment:"you are going to do this again?"

Anya, Through the Looking Glass by Lewis Caroll 'I'll whisper it,' said the Messenger, putting his hands to his mouth in the shape of a trumpet and stooping so as to get close to the King's ear. Alice was sorry for this, as she wanted to hear the news too. However, instead of whispering, he simply shouted, at the top of his voice, 'They're at it again!' 'Do you call that a whisper?' cried the poor King, jumping up and shaking himself." Алиса в Зазеркалье, перевод Демуровой. - Лучше я шепну вам на ухо, - сказал Гонец и, поднеся руки трубкой ко рту, нагнулся к Королю. Алиса огорчилась - ей тоже хотелось знать, что происходит в городе. Но Гонец гаркнул Королю прямо в ухо: - Они опять взялись за свое! - Это, по-твоему, шепот? - вскричал бедный Король, подскочив на месте и передергивая плечами. the complete expression is: "принялся за своё" "взялся за своё" There is a similar expression "взялся за старое" with a bit different meaning (started doind something he is known to do in past). Принялся работать - started working.. Взялся работать - meant "started working" too.... but now mostly means.... I'm not sure how to say it in English. "set about"? "tackle"? When you say, you need someone to write an article and I say "I will do". Of course the verb is normally more specific than 'to work'. Взялся за работу. - Began working, started doing some job, started working on this - or also "took a job". Принялся за работу. - Started doing some job.

The grammar explanation: "Cтарое" means something like 'old things', 'what is old'. "за своё" means doing 'what is his/her/yours/mine', 'things peculiar to him'. It is a substantivated andjective, that is an adjective functioning as a (mass) noun. The usage is facilitated by: - neuter is an abstract gender in Russian ("all". "everything" are neuter. Что это такое? is also neuter). - full forms of adjectives have a demonstrative pronoun hiding inside and sometimes may mean "old one" rather than "old". "за" as you see, just comes after "взялся" (literally 'took') before a noun. "опять за своё!" - is very idiomatic and colloquially commonly used without a verb. Can be used with a verb too. "Опять ты за старое!" - often verb is not omitted. But can be. Translation is "at it again".

Опять за свое - человек повторяет какие либо действия совершенные им в прошлом, чаше всего фраза используется в негативном смысле.