The origin of Flemish is self-explanatory - a reference to the bronchial congestion which characterises the inhabitants of this low-lying, marshy terrain and which led to the guttural, throat-clearing peculiarities of the language.
Walloon or Walon, takes its name from its Welsh origins. It was carried to the continental mainland by migrant Welsh bulb growers following the daffodil blight of 1613-15. Even now, the current standardisation of the spellings of the several distinct Walloon dialects is known as 'Rhonda Walon', while the evidence of Welsh names can still be found in the tulip-growing areas of the Netherlands (Willems, Johannes, Van Rhys, etc).
Frisian developed along the North Sea coast - the continental 'frieze'. Pliny the Younger reported that the Frisians lived on turps; this may explain the anglo-saxon qualities of the language, which bears a close resemblance to the Low English of Romney Marsh, Britain's own nether land [cf: 'Anglo-Frisian Fricatives of Dungeness and New Romney'; Ivan I. Deare; School of Sport and Exercise Science, Loughborough University, 1981].