From Byron's Don Juan:
One with her flush'd cheek laid on her white arm,
And raven ringlets gather'd in dark crowd
Above her brow, lay dreaming soft and warm;
And smiling through her dream, as through a cloud
The moon breaks, half unveil'd each further charm,
As, slightly stirring in her snowy shroud,
Her beauties seized the unconscious hour of night
All bashfully to struggle into light.
This is no bull, although it sounds so; for
'T was night, but there were lamps, as hath been said.
A third's all pallid aspect offer'd more
The traits of sleeping sorrow, and betray'd
Through the heaved breast the dream of some far shore
Belovéd and deplored; while slowly stray'd
(As night-dew, on a cypress glittering, tinges
The black bough) tear-drops through her eyes' dark fringes.
I immediately thought that this meant bullshit, but then I consulted dictionaries and discovered that bullshit was coined in the 20th century. Yet it clearly means "false information" here.
If not from bullshit, what does it derive from?