One way to hold this side of the word: it is an answer to the question what sort of head is on this? Put a number in front and you get how many — a two-headed snake. Put a colour or a shape in front and you get what is growing up there — red-headed, curly-headed. Move the question from the outside of the head to the inside and you get what sort of thinking is in there — level-headed, hot-headed. And because the top of a sheet of paper is also called its head, the same question can be asked of paper: headed paper is paper whose top has already been printed.
Wiktionary gives this side as head (noun) + -ed, with head going back through Middle English heed and Old English heafod.
資料把這一邊拆成 head（名詞）加 -ed，head 再往上追到中古英文 heed、古英文 heafod。所以 -ed 在這裡不是過去式，而是「帶著、有」的意思。
Wiktionary entry *headed*, etymology 1, head (noun) + -ed (packet evidence, source_ref kaikki:3f6f69a190036ae4:1157028010)
Two branches
頭是什麼樣子的
三個意思都在回答同一個問題，只是答案從數量，走到外表，再走到腦袋裡面。
adjective · B1
Used at the end of a compound word to say how many heads something has: a two-headed monster in a story has two heads, a three-headed dog has three, and a many-headed creature has more than you can count.
（複合字）有幾顆頭的
紙的那個「頭」
英文把一張紙最上面那一塊也叫 head，所以「它有什麼樣的頭」這個問題可以原封不動地問到紙身上。
adjective · B2
Of writing paper: already printed at the top with a name and address, so that a letter written on it clearly comes from that person, school or company. British speakers say headed paper or headed notepaper.
（紙）印有抬頭的
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。
headedadj詞源 1/ˈhɛdɪd/
Of a sheet of paper: having the sender's name, address, etc. preprinted at the top.
Having a head or brain with specified characteristics.
Having hair of a specified color.
Having been headed: having had a head formed (by deformation such as die-stamping).
headedadj詞源 2/ˈhɛdɪd/
Heading in a certain direction.
這個拼法也被記錄為下列字的變化形：
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。