It's a term from George Orwell's novel 1984. In the Ministry of Truth, every office had a chute that went straight to an incinerator, and when history was revised -- as it was constantly -- every document that contained a reference to the former version went straight down the chute. The slang term for the chute was "the memory hole." Some "memory holing" means erasing something that happened and substituting a fictional account that doesn't include it.