A man who is his own lawyer has a fool for a client
This proverb is based on the opinion, probably first expressed by a lawyer, that self-representation in court is likely to end badly. As with many proverbs, it is difficult to determine a precise origin but this expression first began appearing in print in the early 19th century. An early example comes in The flowers of wit, or a choice collection of bon mots, by Henry Kett, 1814:
...observed the eminent lawyer, "I hestitate not to pronounce, that every man who is his own lawyer, has a fool for a client.
See also: the List of Proverbs.