Both words in the phrase kith and kin are of Old English origin. Kith is from cȳð (ð is pronounced as a th in modern English), which means known and also crops up in an entirely different spelling the the word uncouth: unknown, unfamiliar, wild, solitary. Kith therefore means one’s acquaintances, as opposed to one’s kin, which comes from the Old English word cynn, meaning family, race, stock. The phrase is first attested in William Langland’s moral allegory Piers Plowman (1377):

How riȝtwis men‥Fer fro kitth and fro kynne yuel yclothed ȝeden.

How rightful men, far from kith and from kin, were clothed wretchedly.