Most people only look up words in a dictionary when they don't know what those words mean. However, looking up familiar words in a dictionary can often be more interesting, and can lead you up all sorts of different paths.
Today I was thinking of the word 'whimsy'. I know what it means but decided to look up its origin. Under 'Etymology' the OED referred me to the entry for 'whim-wham', a word I did not know. The Dictionary said that the history of the 'whim' group of words (whim, whimsy, whimsical etc) is not clear. A Scandinavian origin has been suggested, since the Old Norse word hvima meant 'to wander with the eyes as with the fugitive look of a frightened or silly person', and hvimsa meant 'to be taken aback or discomfited', and so there is a tenuous link in meaning between these words and the current meaning of 'whim' (fantastic notion or odd fancy).
However, the Dictionary goes on to say that 'whim-wham' was the first of the 'whim' group to be recorded, and it appeared roughly at the same time (16th century) as other reduplicative words with 'trivial' or 'frivolous' meanings -- flim-flam, jim-jam, skimble-skamble and trim-tram. Whim-wham was perhaps, therefore, coined on the model of these other words. The Dictionary finishes by saying that 'whimsy' may be related to 'whim-wham' in the same way that 'flimsy' is related to 'flim-flam'.
Whim, as well as meaning a capricious notion or fancy, also has quite a different meaning, namely a machine for raising water or ore from a mine. The OED tells us that this change in meaning is similar to the Engine/Gin model. Gin meaning machine (as cotton gin) developed from the Old French word engin, engine, thanks to the linguistic process known as aphesis, where the initial unstressed sound or syllable of a word gradually disappeared over time (as acute/cute and Egyptian/gypsy). The word gin meaning the alcoholic drink is not related; this comes from genever, a Dutch spirit originally distilled from juniper berries (I've written on gin before).
Yet another meaning of 'whim' or 'whim-wham' can be found in Eric Partridge's old Dictionary of Historical Slang (but not in the OED). If you know French, you'll know that most question words begin with 'qu' (qui, quand, quoi etc), whereas in English most begin 'wh' (who, what, when etc). Sure enough, Partridge tells us rather quaintly that 'whim'/'whim-wham' meant 'the female pudend' in the 18th century.