The Collaborative International Dictionary
Curtail dog
Curtail dog \Cur"tail dog`\ (d[o^]g`; 115). A dog with a docked tail; formerly, the dog of a person not qualified to course, which, by the forest laws, must have its tail cut short, partly as a mark, and partly from a notion that the tail is necessary to a dog in running; hence, a dog not fit for sporting.
Hope is a curtail dog in some affairs.
--Shak.
Wiktionary
curtail dog
n. 1 (context obsolete English) A dog with a docked tail; formerly, the dog of a person not qualified to course, which, by the forest laws, must have its tail cut short, partly as a mark, and partly from a notion that the tail is necessary to a dog in running. 2 (context obsolete by extension English) A dog not fit for sporting.