An Old English pronoun — ilca — is the predecessor of the modern noun ilk. The old pronoun somehow fell away as the English language developed. It does not exist in most dialects of modern English.
Old English, the language of the Anglo-Saxons that was used until about the year 1150, is a highly inflected language with a largely Germanic vocabulary, very different from modern English.
The modern word ilk is synonymous with same. In Scots and then American English it is used in the phrase of that ilk, meaning “of the same place, territorial designation, or name.” It is used chiefly in reference to the names of land-owning families and their estates, as in “the Guthries of that ilk,” which means “the Guthries of Guthrie.”
Centuries ago a misunderstanding arose concerning the Scots phrase: it was interpreted as meaning “of that kind or sort,” a usage that found its way into modern English. Ilk became established in English with its current meaning and as part of speech in the late 18th century.