Rub (b. Fo.A. 18) was the eleventh child, and youngest daughter, of Samwise Gamgee and Rose Cotton.
Ruby follows the custom of noble Hobbit women to be named after jewels, like Ruby Bolger. The jewel ruby derives from Latin rubeus "reddish".
Ruby is the first of the Gamgee family to take such a name, but her father was already a Mayor of the Shire.