Answer:
A Company That Makes Everything
That's typically the response you'll hear.
I've also heard it comes from the Greek word ακμή, which basically means "best" or "prime"... Thus perhaps the company ACME is coined ironically since what it tends to sell do not work, or backfire. ACME, despite being all capitals, is not an acronym. ACME is named for the word acme, which is "the highest point, as of achievement or development." ACME is written in all capitals for name recognition. That is what people see on parcels and crates and it is what is expected. ACME does not stand for "Anvil Company Marketing Everywhere" or "American Company Making Everything" or "A Company Making Everything." ACME does not make everything. At least not yet.