The Welsh word is 'cysgu' meaning 'to sleep'. It's only coincidence here that 'sleep' and 'seep' are one letter apart.

This is all about the drowning of a large section of the Afon Tryweryn valley to provide a reservoir for water supply to Liverpool. The debate started in 1956; legislation to enable it was passed in 1957; and work was completed in 1965 (via many protests and several explosions).

In 2005, Liverpool Corporation unanimously agreed an apology for the works, which have become a symbol of the arrogant disregard for Wales and Welsh culture, and why Meic Stevens's song remains so popular and retains such potency.

In the Tryweryn valley was the community of Capel Celyn, as well as individual farms and cottages. The song is, I think, intending to express the idea that it's not people who now sleep in the drowned houses, but the water, with the overtones that the water is somnolent and dark, in contrast to the vitality and brightness of the community that once lived there.

Lhiats,

Bobby Bob, Ellan Vannin
(Robert Probert, Ynys Manaw)