It is certainly true that palm oil is a highly-valued staple among the Igbo. It is used as lamp oil and for religious sacrifices, as well as for cooking. Food is both cooked in palm-oil and served with it as an accompaniment. There also appear to be different types of palm oil suitable for different dishes.
In context, however, the phrase "proverbs are the palm-oil with which words are eaten" has a more specific meaning than just enhancing the flavor of the conversation. Okoye has come to visit Unoka on the eve of taking the Idemili title. At first, he speaks plainly, but then he begins to talk in proverbs. It is at this point that Achebe compares proverbs to palm-oil. He then says that Okoye spends some time skirting round the subject with his proverbs before asking Unoka to repay the 200 cowries he borrowed from Okoye two years before.
Palm-oil, therefore, is not only useful for adding savor to food or for cooking it. It is taken with food...
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