He unsuspectingly accepted the stranger's invitation and ended up losing his wallet.

unsuspectingly 是副词，由形容词 unsuspecting 加上副词后缀 -ly 构成。unsuspecting 由前缀 un-（表示"不、无"）和 suspecting（怀疑的）组成，字面意思是"不怀疑的"。unsuspectingly 表示"毫无戒心地、不设防地"。