This is from the Latin word "conus", meaning a wedge, or a peak, or basically anything unnecessarily pointy.
If you sleep on the ground then a cone can be a pine cone, a fir cone, a spruce cone, a twig, a stone, a shard of bark, a piece of glass, a rusty nail, teeth of a dead animal, or anything that keeps you from getting the rest you need, no matter how thick your sleeping pad is.
If you happen to sleep on unnoticed animal poo, well that stuff will not keep you awake the first night because it is so soft. (Mmmm, soft.) But you will find it the next day and just the thought of it will prevent you from sleeping for the next few nights. At least.
You get extra points if you manage to pack up in the morning without realizing that your new poo buddy is there, and then get the stuff smeared all over the inside of your pack and everything you carry there.
You get lifetime champion status points if you put your pack in some nice soft poo while setting up camp after sundown and then use your pack under your knees all night, and triple lifetime champion status bonus points if you do this while using a backpacking hammock, and manage to smear poo all over the inside of the hammock, the outside of your sleeping bag, your jammies, and of course have it all over your pack too.
Guess who did that once?
Source: How to talk in the woods.
We few, we grumpy few, we rumply-hat geezers say to you Effort or Eff it. No sniveling then.