During ancient times there was a realization that many of the words that were used in the Veda were becoming difficult to understand. The Nighaṇṭu was a response to that and is a very ancient collection of Vedic words whose meanings were already obscure by that time.
But the Nighaṇṭu is only a collection. Part of the collection is in the form of groups of synonyms, so it is possible to infer the meanings of the words. However other parts of the Nighaṇṭu are just lists of words. Some of these words (about a quarter of them) have been explained in the Nirukta of Yāska which is a commentary on the Nighaṇṭu.
A brief introduction to the Nighaṇṭu can be found here. You can also browse through the collection – it is still in the making, though. Chapters 1 and 5 are complete and the others will be done soon. But even a cursory browsing through the collection will reveal interesting things.
A good example is the much misunderstood and misinterpreted word गो (declined गौ , गावौ , गावः). Nowadays the word generally means a member of the bovine species (cow or bull); however in the Veda it has several other meanings. You will find it listed among the synonyms of Earth (Nighaṇṭu 1.1), Ray of light (Nighaṇṭu 1.4), and Speech (Nighaṇṭu 1.11) which goes to show that the meaning has to be interpreted depending on the context.
An interesting example is the word शची listed in Nighaṇṭu 2.1 among names for कर्म (action, deed). In the Veda, Indra is often referred to as शचीपतिः – lord of (mighty) deeds. But because of the word पति (which also means “husband”) in this compound, later mythology invented a wife for him by the name of शची !