Merry was a kind of butler to the king. For the Saxon and Frankish kings, on whom the Rohirrim were inspired, it was customary to give such titles: steward, stablemaster, butler. 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.