1. 心虚的；缺乏信心的
If you are unsure of yourself, you lack confidence.

2. 不确定的；不清楚的；拿不准的
If you are unsure about something, you feel uncertain about it.

1. lacking or indicating lack of confidence or assurance

e.g. uncertain of his convictions
unsure of himself and his future
moving with uncertain (or unsure) steps
an uncertain smile
touched the ornaments with uncertain fingers

Synonym: uncertainincertain

2. lacking self-confidence

e.g. stood in the doorway diffident and abashed
problems that call for bold not timid responses
a very unsure young man

Synonym: diffidentshytimid