to thrash out

安倍首相が正式にTPP交渉参加を表明しました。

Abe says Japan cannot afford to be left out of TPP talks

Prime Minister Shinzo Abe said Friday that Japan will take part in talks aimed at forging a huge free trade agreement that could involve 40% of the world’s economy.

Abe said Japan could not afford to miss negotiations on thrashing out the Trans Pacific Partnership (TPP).

“I have decided to join negotiations for the TPP, the Trans-Pacific Partnership,” Abe told a news conference. “The vast Pacific Ocean is becoming an inland sea for a huge economic bloc.”
http://www.japantoday.com/category/politics/view/abe-to-announce-decision-to-join-tpp-talks-at-6-p-m-news-conference

thrashは「たたく」・「打つ」などの意味で有名ですが、記事からthrash outというPVを拾いました。
文意から「議論する」という意味で使われているのは推測できます。LDOCEでは、" to discuss something thoroughly with someone until you find an answer, reach an agreement, or decide on something"とあり、「答えを出すまでや同意に達するまで」と物事を「徹底的に解決するために議論をする」という強い表現であると感じました。(Minnesota)