完全陌生：a complete stranger
解析：
complete [kəm'pli:t]
adj.完整的；完全的；彻底的
vt.完成
[ 过去式completed 过去分词completed 现在分词completing ]
stranger ['streidʒə]
n.陌生人；外地人；局外人
例句
1.She was accosted by a complete stranger.
有一个她完全不认识的人过来和她搭讪.
2.He was a complete stranger,but I knew I could trust him.
他对我来说是个完全的陌生人,但是我知道我可以信任他.