off-guard

from WordNet (r) 3.0 (2006)

off-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}]

from The Collaborative International Dictionary of English v.0.48

off-guard \off-guard\ pred. adj.
   not watching for possible danger; not alert; as, caught in an
   off-guard moment.
   [WordNet 1.5]