Other forms: soapily; soapiest
Things that have soap in (or on) them are soapy, and things that resemble soap because they're frothy or foamy are also soapy.
Your cat won't be happy if you pet him with your soapy hands, and you won't be happy if your hot chocolate tastes vaguely soapy. Another way to use this adjective is to mean "overly flattering and fake." The soapy compliment you give your teacher will be just as distasteful to your classmates as that soapy hot chocolate was to you.