'Thrash something out' can mean to discuss a situation thoroughly in order to reach a solution.

For this you might say, "flesh out", but "trash out" wouldn't make any sense in English (except when you take the trash/garbage out). Other good words are "deliberate" (She needs more time to deliberate that idea) or "contemplate" (I'll contemplate this and get back to you).

I also think you mean 'to hash'. The words thrash or thresh would not work here either.

Another phrase that fits your description is "hash out". Here's its definition from Dictionary.com. "hash" ... 8. to discuss or review (something) thoroughly (often followed by out): - "They hashed out every aspect of the issue."

Perhaps you're thinking of "flesh out", meaning to make something more substantive, to elaborate on something. As far as I know, "trash out" isn't an expression, except if you're saying something like "take the trash out", meaning to either literally place garbage outside of the house for collection, or figuratively to remove something (or someone) which is unpleasant. Here's an example for "flesh out". "I've got a basic idea for my travel plans, but I'm going to speak to an agent so they can help me flesh them out." i.e. so they can help me fill in the details, know exactly where I will be on which dates. I hope that helps! Have a good day.