The Collaborative International Dictionary of English v.0.48:

Nap \Nap\ (n[a^]p), v. i. [imp. & p. p. Napped (n[a^]pt); p.
   pr. & vb. n. Napping (n[a^]p"p[i^]ng).] [OE. nappen, AS.
   hn[ae]ppian to take a nap, to slumber; cf. AS. hnipian to
   bend one's self, Icel. hnipna, hn[imac]pa, to droop.]
   1. To have a short sleep; to be drowsy; to doze. --Chaucer.
      [1913 Webster]

   2. To be in a careless, secure state; to be unprepared; as,
      to be caught napping. --Wyclif.
      [1913 Webster +PJC]

            I took thee napping, unprepared.      --Hudibras.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Napping \Nap"ping\, n.
   1. The act or process of raising a nap, as on cloth.
      [1913 Webster]

   2. (Hat Making) A sheet of partially felted fur before it is
      united to the hat body. --Knight.
      [1913 Webster]

   3. The act or process of having a short sleep; as, napping is
      a common occupation for cats.
      [PJC]

WordNet (r) 3.0 (2006):

napping
    adj 1: not prepared or vigilant; "the blow caught him napping";
           "caught in an off-guard moment"; "found him off his
           guard" [syn: napping, off-guard(a), off guard(p),
           off one's guard(p), off his guard, off her guard,
           off your guard]