Gennēma (γέννημα) refers to offspring or that which has been born — the product of generation. It is used both for agricultural fruit (the produce of the vine) and for people (offspring of vipers). The word emphasizes the relationship between source and product: what you are produces what you bear.
Jesus's use at the Last Supper — "I will not drink again of this fruit (gennēma) of the vine until that day when I drink it new with you in my Father's kingdom" (Matthew 26:29) — transforms the ordinary fruit of the vine into an eschatological sign. The cup points forward to the wedding banquet of the Lamb. Meanwhile, the phrase "brood (gennēmata) of vipers" (Matthew 3:7; 12:34) underscores that character reproduces itself in offspring.