The fictional name Llareggub bears some resemblance to many actual Welsh place names, which often begin with Llan- (meaning church), but the name was actually derived by reversing the phrase "bugger all". In early published editions of the play it was often rendered (contrary to Thomas's wishes) as Llaregyb or similar. A double-g is not used in written Welsh. If the name were to be pronounced in Welsh as spoken in South Wales, it would be .
The geographical inspiration for the town has generated intense debate. Laugharne was the village where Dylan Thomas lived intermittently from the 1930s. This town was probably the inspiration for the people of Llareggub, although the topography of the town is thought to be based on New Quay, Ceredigion where Dylan was staying when he started writing the play seriously in 1944. Both towns use the Under Milk Wood association to attract tourists, hence the rivalry, and The Dylan Thomas Trail has been opened in New Quay.
More recent research has indicated that most of the first half of the play was written in South Leigh, Oxfordshire, whilst the second half was mostly written in America in May 1953. Fewer than 300 lines were written in Laugharne.
Dylan drew a sketch map of the fictional town. This is now at the National Library of Wales and can be viewed online.
The name Llareggub was first used by Dylan Thomas in a short story The Burning Baby published in 1936. ('Death took hold of his sister's legs as she walked through the calf-high heather up the hill... She was to him as ugly as the sowfaced woman Llareggub who had taught him the terrors of the flesh.')
In the play, the Rev Eli Jenkins writes a poem, which describes Llareggub Hill and its "mystic tumulus". This was based on a lyrical description of Twmbarlwm's "mystic tumulus" in Monmouthshire that Thomas imitated from Arthur Machen's autobiography Far Off Things (1922)
The town's name is the inspiration for the country of Llamedos (sod 'em all) in Terry Pratchett's Discworld. In this setting, Llamedos is a parody of Wales.
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