steepen

verb

Etymology: From steep + -en. Compare Old English stīepan (“to steepen”).

To make steeper.
“With a rare and beautiful ease one can now ring up a boffin, as I did the other day, and say: "As a price for not opposing our Parliamentary Powers for a new marshalling yard, the Council at X demands that the bridge over X Lane shall have 16 ft. 6 in. headroom. This means steepening our gradient from 1 in 70 to 1 in 65 for half a mile on a 20-chain curve. What difference will this make to the loads of Type "2", "3" and "4" diesels please?". Back comes the answer.”
To become steeper.
“As the way steepened […] I could detect in the hollow of the hill some traces of the old path.”
“Soon after Crowhurst, the down grade steepens to 1 in 100 as the line descends the side of a valley, at the mouth of which a glimpse of the sea can be caught—the only one.”