The many-to-one correspondence between Guwal [everyday language] and Jalnguy ["mother-in-law" language] vocabularies was a key to the semantic structure of Dyirbal. If one Jalnguy word was given as the equivalent for a number of distinct Guwal terms, it meant that the Guwal words were seen, by speakers of the language, to be related. For nouns, it revealed the botanical and zoological classifications which the Aborigines perceived. For instance, bayi marbu "louse", bayi nunggan "larger louse", bayi daynyjar "tick", and bayi mindiliny "larger tick" were all grouped together under a single Jalnguy term, bayi dimaniny.After I returned from doing fieldwork in Papua New Guinea, the three of us who had spent most of 1976 living in little villages along the north coast of New Guinea were invited to share our experiences at a Q&A session for other linguists. At one point, we were asked how our linguistics training had helped prepare us for our fieldwork experiences. I answered something along the following lines: "Well, it substantially increased my boredom threshold, and that proved extremely useful in the field."
It could be even more revealing with verbs. The everyday style has four different words for kinds of spearing, and also such verbs as nyuban "poke a stick into the ground (testing for the presence of yams or snails, say)", nyirran "poke something sharp into something (for example, poke a fork into meat to see if it is cooked)", gidan "poke a stick into a hollow log, to dislodge a bandicoot". All seven of these Guwal verbs are rendered by just one word in Jalnguy: nyirrindan "pierce".
UPDATE: Part III, the exciting conclusion here.