give ground

Trouble is My Businessを読んでいます。
I leaned against the closet door. It didn't move. I threw more weight against it. It still didn't move. braced against it I pushed the bed away with my foot, gave ground slowly.
A weight pushed against me hard. I had gone back a foot or so before anything else happened. Then it happened suddenly. He came out-sideways, in a sort of roll. I put some more weight back on the door and held him like that a moment, looking at him.
He was still big, still blond, still dressed in rough sporty material, with scarf and open-necked shirt. but his face wasn't red any more.
I gave ground again and he rolled down the back of the door, turning a little like a swimmer in the surf, thumped the floor and lay there, almost on his back, still looking at me.
"gave ground" の意味がピンとこないので辞書で "give ground" を見ます。
・Oxford English Dictionary: Retreat or lose one’s advantage during a conflict or competition: he refused to give ground on this issue
・Collins Dictionary: to draw back or retreat: But as more and more men crossed the drawbridge, the castle defenders were forced to give ground.
意味は分かりましたが、何故この意味が出てくるのだろうと不思議に思いましたが、次の辞書に説明がありました。
・The American Heritage Dictionary of Idioms: Yield to a stronger force, retreat, as in He began to give ground on that point, although he didn't stop arguing entirely. This expression originated in the 1500s, when it alluded to a military force retreating and so giving up territory to the enemy. By the mid-1600s it was being used figuratively.
なるほど、陣地を渡す（取られる）ことなのですね。