You can picture assure as taking someone's worry off their shoulders by telling them, firmly, 'it's fine, this is true / this will happen' — you put your own credibility behind the words so they can stop fretting and feel safe.
From the word’s history
From Old French asseurer, from Vulgar Latin *assecurare = ad- ('to') + securus ('free from care', se- 'free from' + cura 'care'). So the root literally means 'bring someone to a care-free state' — which is exactly the modern core: talk someone out of their worry.
源自古法語 asseurer，再往上是通俗拉丁文 *assecurare，由 ad-（朝向）＋ securus（無憂無慮）組成；securus 又拆成 se-（脫離）＋ cura（掛念）。所以字根的字面意思就是「把某人帶到沒有掛念的狀態」，恰好對上現代的核心：把別人講到放下心來。
Online Etymology Dictionary, https://www.etymonline.com/word/assure
登入就可以看每一支怎麼連回核心畫面。
原始詞典資料4 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，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。
assureverb/əˈʃɔː(ɹ)/
- 1
To make sure and secure; ensure.
- 2
To give (someone) confidence in the trustworthiness of (something).
- I assure you that the program will work smoothly when we demonstrate it to the client.
- He assured of his commitment to her happiness.
- 3
obsoleteTo guarantee, promise (to do something).
- That as a law for euer should endure; / Which to obserue in word of knights they did assure.
- 4
To reassure.
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。