Snive is a made-up word, which came into creation somewhere around 1985. It has several meanings, but the most common one is to steal or pilfer. For example, "I snived a slice of cake before dinner."
You can snive, you can be sniving, you might have snived, or you can be a sniver.
An additional definition is in the form, "he snived her," which could mean that he cheated her in some way. It also can refer to the act of sex.