The term threading is used to indicate a certain type of conversation viewing, particularly for message boards, Usenet, and email conversations, that tries to make the conversation easier to visualize by organizing the messages in a tree hierarchy. Threading is really best used with inline (or "interleaved") posting style on messages, as the tree hierarchy is vital to retrieving trimmed content from the interleaved messages.
A lot of modern email tools (eg: Microsoft Outlook) by default do not thread, and instead encourage users to engage in top-posting, which keeps all context in every message. Some people may refer to the context inside a top-posted email as a "thread", but one could argue that technically that isn't really a thread, and "string" is probably as good an alternate word for that as any other. Then again, if it "sounds stupid" then perhaps not. You could also argue that any path through the tree is a "thread", so that's still the proper word. (Personally I'd prefer to solve this quandary by not using top-posting, but I seem to be losing this particular battle).
The first threaded newsreader was nn in 1984, by Kim F. Storm. Ngrams couldn't find a reference for "threaded newsreader" prior to 1989, but I know firsthand the term was in use in the mid-80's on Usenet.