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Horseback is the back of a horse, but you almost always meet it in the phrase on horseback — riding a horse. Police on horseback. They crossed the river on horseback. American English also says horseback riding for the activity of riding. Though actually, you do not say a horseback the way you say a horse, and you do not say on a horseback. The little word on takes horseback with no a. It is about riding, not about sitting in a cart that a horse is pulling — that cart would be horse-drawn, and you would be in the cart, not on horseback.
馬背上（多用於 on horseback：騎著馬）
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
horsebacknoun/ˈhɔɹsˌbæk/
- 1
The back of a horse.
- They came on horseback across the plains.
- 2
A ridge of sand, gravel, and boulders.
horsebackadv/ˈhɔɹsˌbæk/
- 1
On the back of a horse.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。