A blatherskite may either be noisy talker of blatant rubbish or the foolish talk or nonsense that such a person spouts. It's actually a Scots word, really a pair of words, known from the seventeenth century on. These days, though, it's more American than either British or Scots. That came about through one of those curious accidents of linguistic history that make the study of etymology such fun.

Both halves of the word seem to be from Old Norse. Blether is a Scots word meaning loquacious claptrap, which comes from Old Norse blathra, to talk nonsense; it exists in various forms now, such as blather or blither (if you call someone a blithering idiot, as people in Britain often did in my youth, you're using the same word, though most of the meaning had by then been leached out of it). Skate (skite, as Australians and New Zealanders will know it) is more problematic, but is the Scots word for a person held in contempt because of his boasting, which may derive from an Old Norse word meaning to shoot (and, if true, is probably the origin of the American skeet, as in skeet shooting, so that phrase actually means "shoot shooting").

Blatherskite is first recorded in an old Scots ballad called Maggie Lauder, attributed to Francis Sempill (or Semple) and dated about 1643, still well known today. There are various transcriptions of the first verse, one being:

Wha wadnae be in love
wi' bonnie Maggie Lauder?
A piper met her gaun tae Fife
and speirt what was't they ca'd her.
Right dauntingly she answered him,
"Begone ye hallanshaker.
Jog on your gate ye blether skyte,
my name is Maggie Lauder".

A rough translation into modern English is:

Who wouldn't be in love
with beautiful Maggie Lauder?
A piper met her going to Fife
and asked what people called her.
Discouragingly she answered him,
"Go away, you vagabond!
Be on your way, you talkative boaster,
my name is Maggie Lauder".