MERCY is a machine dedicated to Offensive Security for the PWK course, and to a great friend of mine who was there to share my sufferance with me. :-)
MERCY is a name-play on some aspects of the PWK course. It is NOT a hint for the box.
If you MUST have hints for this machine (even though they will probably not help you very much until you root the box!): Mercy is: (#1): what you always plead for but cannot get, (#2): a dubious machine, (#3): https://www.youtube.com/watch?v=c-5UnMdKg70
Note: Some report a kernel privilege escalation works on this machine. If it does, try harder! There is another vector that you should try!
Feel free to contact the author at https://donavan.sg/blog if you would like to drop a comment.