WordWeb Online

1. Lacking self-confidence
   "a very unsure young man";
   - diffident, shy, timid
    
2. Lacking or indicating lack of confidence or assurance
   "unsure of himself and his future"; "moving with unsure steps";
   - uncertain, incertain [non-standard]
    
3. Not established beyond doubt; still undecided or unknown
   "an unsure future";
   - uncertain

See also: ambivalent, assurance, authority, certainty, confidence, doubtful, dubious, foregone conclusion, groping, indefinite, indeterminate, self-assurance, self-confidence, sure thing, sureness, undetermined, up in the air

Antonym: sure