Sexy Pistol - this would be a gun which is very sexy looking Sex Pistol - this would be a gun which is used for sex A student book - this a book which is used by a student during the school course A student´s book - this is a book which belongs to a student The adjective in English can drastically change the meaning when combined with a noun :)

Sex pistols, as I am sure you know, is a band. The name is slang for a penis (or I guess technically more than one penis). You can't substitute "sexy" because that would be modifying the word "pistols". So to say "sex pistol" is to refer to a penis, but if you said "sexy pistol", you would be saying that you thought a gun (pistol is another name for a gun) was sexy.