Perhaps from flanker 'someone who stands beside you' (18-19 centuries), from FLANK1
flunkey
noun
flun‧key, flunky [countable]informal
someone who does small jobs for an important person, especially someone who does this because they are trying to please the person - used to show disapproval:
One of his flunkeys let me in.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.