in an underhand mannerexprexpression: Prepositional phrase, adverbial phrase, or other phrase or expression--for example, "behind the times," "on your own."
(in a deceitful or dishonest way)
偷偷摸摸地 tōu tōu mō mō de
以欺诈的方式 yǐ qī zhà de fāng shì
以不诚实的方式 yǐ bù chéng shí de fāng shì
He is behaving in an underhand manner, going out with his best friend's girlfriend without him knowing.