To blather is to talk foolishly at length. Blather on, stop blathering, endless blather.
The length is as much the complaint as the foolishness — blathering is talk that will not stop. It is informal and mildly contemptuous. Blether is the Scots form of the same word. Note that it came from an Old Norse verb meaning to talk nonsense, which is exactly where it stayed.
1verbblatherTo talk foolishly at length.
“He blathered on about his holiday while everyone else tried to watch the match.”
blather is a foolish talking.
Old Norse blathra means to talk nonsense. The Scots blether is the same word.
Old Norse
blaðra — to speak inarticulately, talk nonsense
Middle English
bletheren
English, today
blather
From Middle English bletheren, bloderen, from Old Norse blaðra (“to speak inarticulately, talk nonsense”). Cognate with Scots blether, bladder, bledder (“to blather”), dialectal German bladdern (“to talk nonsense, blather”), Norwegian bladra (“to babble, speak imperfectly”), Icelandic blaðra (“to twaddle”).
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire