The term Garlic Routing has a diverse history of varying interpretations. As it currently stands, Monero defines Garlic Routing as the method in which Kovri and I2P create a Message-based anonymous overlay network of internet peers.
The Garlic Encryption of Garlic Routing is similar to the Layered Encryption of Onion Routing and effectively conceals the IP address of the sender and secures information sent from the sender to its Destination (and vice-versa).
[I think that there was some attempt to come up with a plant whose structure resembled the 'leaky-pipe' topology of tor, but I don't believe we ever settled on one.]
during the free haven brainstorming, there was a moment where we described a routing mechanism, and somebody said "garlic routing!", and everybody laughed. so we for sure thought we had invented the name, at the time.
Note: permission to use the aforementioned quotes was granted by Nick Mathewson and Roger Dingledine
Read more in Garlic Encryption.