The Collaborative International Dictionary of English v.0.48:

non-U \non-U"\ (n[o^]n*[=u]"), adj.
   Not characteristic of the upper classes especially in
   language use. [Chiefly British] [Narrower terms: lower-class
   (vs. middle-class) (vs. upper-class), low-class]

   Syn: vulgar.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

non-U
    adj 1: not characteristic of the upper classes especially in
           language use