If one navigates to the ECDSA Go package page, he can observe that:
This implementation derives the nonce from an AES-CTR CSPRNG keyed by ChopMD(256, SHA2-512(priv.D || entropy || hash))
While I am familiar with AES in CTR mode and SHA2-512, I am not familiar with what ChopMD refers to and I haven't been able to find any definite answer.
The only reference to ChopMD I have found is this PDF, showcasing how to improve ChopMD but never really going into detail as to who conceived it, why etc. MD commonly stands for Message Digest in the hashing functions but within the paper, MD refers to the Merkle-Damgaard construct, which is internally used by MD-5, SHA-2 and SHA-1.
If someone could clear up what ChopMD refers to I would appreciate it.