He meant singing dog, likely conflated due to his linguistic interest.
"Juggling dog" has only been expressed a single time previously in our corpus of humanity:
During the Middle Ages, however, church and state sometimes frowned more sternly on the juggler. "The duties of the king," said the edicts of the Sixth Council of Paris during the Middle Ages, "are to prevent theft, to punish adultery, and to refuse to maintain jongleurs."(4) What did these jugglers do to provoke the ire of churchmen? It is difficult to say with certainty, since the jongleurs were often jacks-of-all-trades. At times they were auxiliary performers who worked with troubadour poets in Europe, especially the south of France and Spain. The troubadours would write poetry, and the jongleurs would perform their verses to music. But troubadours often performed their own poetry, and jongleurs chanted street ballads they had picked up in their wanderings. Consequently, the terms "troubadour" and "jongleur" are often used interchangeably by their contemporaries.
These jongleurs might sing amorous songs or pantomime licentious actions. But they might be also jugglers, bear trainers, acrobats, sleight-of-hand artists or outright mountebanks. Historian Joseph Anglade remarks that in the high Middle Ages:
"We see the singer and strolling musician, who comes to the cabaret to perform; the mountebank-juggler, with his tricks of sleight-of-hand, who well represents the class of jongleurs for whom his name had become synonymous; and finally the acrobat, often accompanied by female dancers of easy morals, exhibiting to the gaping public the gaggle of animals he has dressed up — birds, monkeys, bears, savant dogs and counting cats — in a word, all the types found in fairs and circuses who come under the general name of jongleur.”(5)
-- http://www.arthurchandler.com/symbolism-of-juggling