原以为很老实英文

I used to think you were sick.

honestfrank; good; veracious; veridical; well-behaved

跟中文意思的老实最接近的就是Honest,这是形容词，名词形式是honesty

单纯[dān chún]simple；pure；alone；purely 老实[lǎo shí]honest；frank；well-behaved；good；clean hands ； 天真[tiān zhēn]innocent；artless；naive