steep ...1859 obs. rare
from Latin proclivitas (downward slope, tendency, propensity), from prōclīvis (sloping deeply forwards and downwards) + -ous,
probably after declivitous, acclivitous
FIRST DOCUMENTED USE
1859 - see EXAMPLE below
"...The roads, as usual in East Africa, are tracks trodden down by caravans and cattle, and the watercourse is ever the favourite Pass. Many of the ascents and descents are so proclivitous that donkeys must be relieved of their loads; and in fording the sluggish streams, where no grass forms a causeway over the soft, viscid mire, they sink almost to the knees..."
From: The Journal of the Royal Geographical Society of London
- Sir Richard Francis Burton