actually, speak for yourself is a term used when you are trying to turn attention away from yourself and onto the person who was accusing you. For example, if you are in a group of people and someone says that Andy never goes to class, and yet they never go to class to, you can say "speak for yourself". It basically points out that they are guilty of the flaws they point out in other people

speak for yourself vt. 为自己辩护(谈自己的事) 例句: Speak for yourself, but I think you're totally wrong. 我不同意你的看法，但我认为你完全错了。 "I think we've done enough drinking for one evening." "Speak for yourself, John." "我想我们的酒喝够了。""约翰，别把我也扯进去。" off the wall ph. 【俚】怪诞的, 荒诞的

It depends on the context, but Tel's definition is the more common one. It doesn't always take on an accusatory tone, it may be said in a jokingly. Domasia is also correct, and this phrases may be used to coax an opinion from someone who is under someone else's influence /control.

'speak for yourself' means that someone is asked to tell (/express /utter) his own opinion (/impression /idea /view /point of view). For example: The student and his parents meet the teacher. The teacher addresses his studend and asks him a question. The parents answer for the student (/answer on the student's behalf). The teacher might say to him: 'speak for yourself!' To be 'off the wall' means something crazy, unusual, strange, bizarre. An idea (/a thougt) is 'is off the wall'.

如果直译的话，很简单，为自己说话，远离战争。我的理解。