So, the said verse is one of strangest in the Sermons I think. Can anyone elucidate further on what is meant by "meat"?
My attempt on making sense of the OP's passage is this: meat has many definitions, one of them being, "the essential point or part of an argument, literary work, etc.; gist; crux: The meat of the play is the jealousy between the two brothers." (Dictionary.com). Sophists just means philosophers.
Knowing this, I think the passage is trying to say that all language is based on having an important point to it, such as simply telling another person something important. Language was made to be able to communicate with people and accomplish things or inform them of what you know, or other things such as that. For the "Do not let the sophists fool you" part, because philosophers usually think about things on a much larger scale than normal and try to find deep meaning or truth in anything they question, I think the passage is just warning against believing language to be something bigger than it is, such as thinking of language to be an artistic expression of a people's culture for example (I can't really think of anything to use as a good example). So not letting the sophists fool you could mean not letting people try to tell you language is something more than the actual point of having language (which is to help communicate, etc).
This is just my attempt at it. I usually can see at least some meaning in these kinds of passages when I look up the many definitions of certain words and try putting it together.
I like the theory that language is man-made, false and incapable of actually relaying 'real' concepts(that it's not "something bigger than it is" or that it's only there for language's sake), just because it's true in our world as well. However that is just what the sophists would think and therefore doesn't coincide with not letting
'the sophists fool you'. The sophists were skeptics and believed that if the ultimate truth exists that we can't know it, and thus didn't "try to find deep meaning or truth" as mainstream philosophers did. They focused on practical day-to-day problems (which put them at odds with the school of philosophy); so we're probably dealing with language being profound rather than mundane...
Therefore, I think that language having a deeper meaning other than just for communication is the actual point, that it has some 'meat' to it or helps with/to rely some ultimate truth (this type of statement is something the sophists would not like). That the Sermon goes on to talk about apology (which may be in reference to apologetics) helps this stance as well...
Also, for the sake of discussion I'd throw out the mundane interpretation that
"all language is based on meat" in the context that all language is just sounds formed by the tongue(meat); an easy rebuttal however is to bring up written language, though since the sophists were teachers of rhetoric (which in their case is the art of persuasive speaking) then the sentence is probably in reference to the spoken word rather than the written word (also since writing employs fingers(meat) it might still hold anyway)...