|Not these crumbs|
According to The Express, which seems to be on a different trajectory than Cambridge (I've formed that conclusion based on the Wikipedia description), "[Crumbs] is one of many [expressions] which originates from using the first few letters of a swear word and substituting a more socially acceptable ending. So Christ becomes crumbs or Christopher Columbus."
I reiterate: I've never heard this expression before, even though I've watched lots of British shows on PBS, and certainly have never heard an American say it. Maybe I should start using it. But then again, people will think I'm spotting some bread or cake crumbs on the floor :p