Merry was a kind of butler to the king. It was customary to give such titles to Saxon and Frankish kings from which the Rohirrim draw their inspiration: butler, stablemaster, steward. Butler means 'bottle-bearer', words very similar to holdwine.
Of course, in a royal court of a medieval king such a position implied much more than being a mere servant: they were positions that denoted being friends of the king and men of trust. Butler and steward came to mean ministerial positions.