The Collaborative International Dictionary of English v.0.48:
Underbred \Un"der*bred`\, a. Not thoroughly bred; ill-bred; as, an underbred fellow. --Goldsmith. [1913 Webster]WordNet (r) 3.0 (2006):
underbred adj 1: (of persons) lacking in refinement or grace [syn: ill- bred, bounderish, lowbred, rude, underbred, yokelish] 2: of inferior or mixed breed; "an underbred dog"