Fidelity's Anthony Bolton - What's a SIV?

Writing an opinion piece for Hargreaves Landsdown, the legendary UK fund manager, made the following claim

"In nearly four decades of investment, I had never come across SIVs - special investment vehicles - until the past 12 months."

Now that sort of ignorance is worrying given he was someone who was running one of the largest UK investment funds for over 10 years.