Sentence:  ... when he asked Harry about singing, the lad broke out with a hymn to the tune of Dr. Martin Luther, which set Mr. Holt a-laughing; and even caused his grand parrain in the laced hat and periwig to laugh too when Holt told him what the child was singing. For it appeared that Dr. Martin Luther's hymns were not sung in the churches Mr. Holt preached at. ¶ “You must never sing that song any more: do you hear, little <WSD>mannikin</WSD>?” says my Lord Viscount, holding up a finger. 
Meaning of the word marked with <WSD> tags:  A little man (sometimes as a term of endearment).