Etymology of "ramp up"?

The idiom "ramp up" is frequently used in English to mean "increase the size/amount of"; for example:

The company ramped up its advertising to try and sell more products.

What, though, it the etymology of this phrase? Why do we use the verb "ramp" here?

There is also the usage "ramp down" in addition to "ramp up"

I think the definition of a ramp as an inclined slope is what is in use in both cases. If the slope inclines upwards, it's "ramping up" and signifies an increase in whatever is being measured. Similarly for "ramping down"

Etymonline for ramp also identifies it as a slope.

1778, "slope," from Fr. rampe

Note: This also says

back-formation from O.Fr. verb ramper "to climb, creep" (12c.)

which is what @Annarita's answer has

However, the usage for a creeping plant I find is not "ramped up" but just "ramped"

(of a plant) grow or climb luxuriantly: ivy ramped over the flower beds.

The verb "to ramp" entered English in the 14th century, and the expression "ramp up" was first applied to climbing plants such as vines. From this usage, "to ramp up" came to mean a general escalation or a rapid increase in activity, a significant increase in the level of output of a company's products or services. A "ramp up" typically occurs in anticipation of an imminent increase in demand.