Wordnet 3.0
ADJECTIVE (1)
1. 
 not prepared or vigilant; 
- Example: "the blow caught him napping"- Example: "caught in an off-guard moment"- Example: "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]
WordNet (r) 3.0 (2006):
off your guard
    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]