If people thrash out a problem or a dispute, they discuss it thoroughly until they reach an agreement. a sincere effort by two people to thrash out differences about which they have strong feelings. = hammer out
discuss vehemently in order to reach a solution or an agreement; "The leaders of the various Middle Eastern countries are trying to hammer out a peace agreement
If people thrash out something such as a plan or an agreement, they decide on it after a great deal of discussion. The foreign ministers have thrashed out a suitable compromise formula = hammer out