Today is St David's Day, St David being the patron saint of Wales. One of the emblems of Wales is the daffodil, so that's the word I'm going to look at today.
Daffodils grow profusely in England and Wales, so presumably they have always existed in Britain, but for some reason the d of daffodil only got added in the 16th century. Before that the flower was known as an affodill. There is a genus of flowers called Asphodelus, and the daffodil/affodil was originally a variant of this flower. The OED says that this added d has not been satisfactorily accounted for. There are various theories: that the process of adding a d is similar to the process that turned Edward into Ted; that it is a shortened form of the definite article 'the' (cf the Yorkshire song On Ilkley Moor bar t'at) (NB t and d are similar sounds - they are pronounced the same way, the only difference is that t is unvoiced and d is voiced); that it is a shortened form of the Dutch or Flemish article de (Netherlands being well known for growing bulbs); or that it is from the French de (fleur d'aphrodille).
Other words in the OED for daffodil are daffodilly and daffadowndilly (each of which can be spelled in various ways).
The daffodil is one emblem of Wales, and the leek is another. Why two quite different things? Well, possibly because they are the same word in Welsh; cenhinen means both 'leek' and 'daffodil'.