I've seen so many plausible and implausible, delightful and mundane expanations of old expressions bunked, debunked, rebunked, junked and defunct, that no longer believe ANY of the explanations. I think Snopes captured the matter well:

"Most of us feel a bit of a glow when we think we're in possession of information others aren't privy to, and when a titillating or apt story is thrown in behind the trivia, these things just take off."

(But - carry on!) (Oh - sailors in the British navy had to tote or "carry on" their personal gear, when boarding a ship. Often, the morning after a last night's spree ashore, a groggy sailor would hesitate to try to determine if he was about to board the right ship. The presiding officer would holler ... hence the commonly used ... blah, blah, blah ... ).