"이따가 잘자" => 이따가 잘 자 => Have a good sleep later. (similar to "Good night", but saying it in advance) - 이따가: later. In a little while. After a bit. - 잘 자: Have a good sleep. Good night. Saying 잘 자 (Good night) by itself is the common way. 이따가 잘 자 sounds a bit odd, but it depends on the context.