Thrash out, a phrasal verb, means to discuss and solve a problem or dispute in detail. The antonyms for "thrash out" could be phrases like ignore, avoid, evade, reject, or dismiss. Ignoring a problem could be an optimal solution when it's not necessary to address it at that time. Avoiding could be appropriate when the solution is already clear, and the discussion doesn't add value. Evading could be the best option when there is no solution insight, and the discussion has reached a deadlock. Rejecting thrashing out could be an alternative in situations where the subject is trivial or not necessary to cover. Dismissing a topic might be beneficial if it's already been discussed or if it doesn't add much value to the meeting.