A person who has become a complete and utter follower of another group or person. He/she will act on their whim without any thought about how stupid he/she will look or the consequences of said actions. They are usually regarded with a mixture of contempt and amusement by those whom they "serve"
"he's not their friend, he's just their monkey butler"
"hey man, get me a beer"
"fuck off, make the monkey butler do it"
A terminology that, if applied in a certain way, can be deemed offensive.
As the guests arrived back at Faversham Retreat, exhausted from a day of Big Game hunting, they were greeted by Lord Fotheringham-Smyth's ever courteous monkey butler, who politely ushered them into the Portrait Room for pink gin and cucumber sandwiches.