From the word’s history
From Old English hundeahtatiġ; eight + -ty.
源自古英文 hundeahtatiġ，就是 eight 加上 -ty。
The number 80 — eight tens. That is the whole word. Everything else on this page is not a new meaning of eighty but the plural form eighties, which takes 80 and stretches it into the ten-year band 80-89: the decade, the age range, the temperature range.
可以想成數線上的 80（八個十）——這個字本身就只到這裡。下面那三個用法都不是 eighty 多出來的意思，而是複數形 eighties 把 80 這一點拉開成 80–89 那一段十格的區間：年份的那十年、年紀的那十歲、氣溫的那十度。看到 eighties，就把「一個點」換成「一整段」。
原始詞典資料1 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。
eightynum/ˈeɪ.ti/
- 1
The cardinal number occurring after seventy-nine and before eighty-one, represented in Roman numerals as LXXX and in Arabic numerals as 80.
- Merryday, who was appointed by President Geroge^([sic]) H. W. Bush, said the complaint contains eighty pages of repetitive claims and praise for President Trump, but fails to establish the two counts of defamation alleged.
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。