Andrew,
a name commonly used in old plays for a valet or man-servant. Probably a Merry Andrew is simply the mirth-making Andrew or domestic jester. (See Merry Andrew.)
Similarly, Abigail is used in old plays for a waiting gentlewoman. (See Abigall.)
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.