The Phrase Finder explains that the term means “Childish and weakly sentimental.” Ambrose Phillips was a poet and tutor to George I’s grandchildren in the early 1700s, and he wrote sentimental poems in praise of the royal children to curry favor. He wrote “…rather affected and insipid nursery language, of the ‘eency-weency’, ‘goody-goody’ sort.” Namby-Pamby was included in one of his poems.