McRib is a hilarious name for a service
(The original McDonald's McRib sandwich is well known for only being sold a limited time.)
So "Mcrouter" comes from Memcache-Router, then the obvious McDonalds jokes are made and someone cleverly suggests "Mcrib" for the next service. But I can't think what the backronym would be for it. Memcache Ring Buffer maybe. Or Broker.