I’m afraid this may sound boastful, but; Sorry, let me brag about something
People use this expression when they want to brag about something but don’t want to sound modest and not too cocky or boastful. This is a very advanced phrase and so not commonly heard in daily conversations.
It is said to originate from the common practice in the old days: people used to proudly talk about how good and unique their homemade miso is.
I’m afraid this may sound boastful, but my book has sold a million copies.