The twenty-second Shire-thain, the ninth descendant of Isumbras Took I to hold that title. He lived at the time when pipe-weed was introduced to the Shire; it was first grown just a few years before Isengrim became Thain. He is perhaps best known as the founder of Great Smials, which was to become the ancestral home of the Took family in the Tookland of the Shire.
In Old English grim has a meaning much stronger than its modern usage. It implied fierceness, or even savagery or blood-thirstiness, so Isengrim 'iron grim' would originally have been a warrior's name. Historically, the name Isengrim appears in medieval literature, where it belongs to a greedy and foolish Wolf in various beast fables; this may have been the ultimate source from which Tolkien selected the name.