The name 'Robin' is difficult to explain in this context, since in reality it developed as a diminutive form of the personal name 'Robert' ('shining glory') and is ultimately French - a linguistic origin not normally found among the Shire-hobbits. Perhaps we're intended to simply take it as representing a generally diminutive name, and the fact that the other Hobbit named 'Robin' (after whom Sam Gamgee's son Robin was possibly named) belonged to the 'Smallburrow' family tends to support this. Alternatively, perhaps this Hobbit-name came from their term for the red-breasted bird known as a 'robin' in English.