ETYMOLOGY
from dialect blather (empty, noisy talk) + bag
EXAMPLE
“… The wife whiles took a taste, and while under its influence was a perfect bletherbag. She hadna a tooth in a’ her head, and on that account her tongue mair room in her mouth; and on ordinary occasions it had little rest. …”
From: Life Studies of Character
By: John Kelso Hunter, 1870
Chapter XXXIV. Breaking the Pledge
Lexicophilia 