笔 bǐ is the picture of a writing brush. The simplified form sets bamboo, the radical ⺮ (zhú), over hair, 毛 (máo): the bamboo handle above and the soft animal-hair tip below, the two materials that make a Chinese brush. The character is a compound of meanings rather than a sound-borrowing, and you can read its parts off the page directly: bamboo for the shaft, hair for the tuft that holds the ink.

The traditional form 筆 tells the older story. It keeps bamboo ⺮ on top but sets 聿 (yù) below, an early pictograph of a hand gripping a brush. 聿 by itself meant "writing brush" in the oldest texts; adding the bamboo radical specified the bamboo-handled instrument. The simplified 笔 swapped that hand-and-brush element for the plainer 毛 (hair), making the same object easier to write while keeping the bamboo-over-tuft logic intact. 笔 and 筆 are the standard simplified and traditional pair for the one word.

From the implement three senses unfold:

1. The writing tool. The brush or pen itself: 毛笔 máobǐ (writing brush), 铅笔 qiānbǐ (pencil), 钢笔 gāngbǐ (fountain pen), 笔尖 bǐjiān (the nib or tip).
2. The act and trace of writing. What the brush does and leaves behind: 笔记 bǐjì (notes), 笔画 bǐhuà (a stroke of a character), 笔顺 bǐshùn (stroke order).
3. A measure word. Borrowed to count sums of money, accounts, and deals, things "written down" in a ledger: 一笔钱 yī bǐ qián (a sum of money), 一笔生意 yī bǐ shēngyì (a business deal).

The thread is the loaded brush touching the page. It is at once the object in the hand, the motion it makes, and, by extension, each entry it sets down in a book of accounts.

Once 笔 names the tool, it slides easily into the writing the tool produces. A single touch of the brush to paper is a 笔画 bǐhuà, a stroke, and every Chinese character is built from a fixed set of them. The order in which those strokes are written is the 笔顺 bǐshùn, the stroke order, which a learner drills until the hand moves on its own. The unique trace your particular hand leaves is your 笔迹 bǐjì, your handwriting.

Pushed further, 笔 stands for written expression as a craft. 文笔 wénbǐ is one's writing style or literary skill, the quality of the line a writer draws across a page of prose. A clumsy or spoiled passage is a 败笔 bàibǐ, a "failed stroke," while a brilliant turn of phrase is praised as a 妙笔 miàobǐ. Here 笔 stands in for the act of writing the way "the pen" does in English idioms about authorship.

The same 笔 that means "a stroke" works as a measure word, and the image holds: each entry in a ledger is one stroke of the pen. Placed between a number and a noun, 笔 counts sums of money, accounts, and transactions: 一笔钱 yī bǐ qián (a sum of money), 一笔账 yī bǐ zhàng (an account), 一笔生意 yī bǐ shēngyì (a piece of business), 一大笔收入 yī dà bǐ shōurù (a large income).

The choice of measure word frames the thing as a single recorded item: money treated as a lump sum rather than loose coins, a deal treated as one written-down transaction. Compare 一些钱 yīxiē qián "some money," which is vague, with 一笔钱 yī bǐ qián, which feels like a definite, accountable amount. When you hear 笔 in front of 钱, 账, 生意, or 债 zhài (debt), it is this ledger sense, not a literal pen.

See the character itself as a brush stood on its tip: bamboo ⺮ at the top is the hollow handle, and 毛 (hair) below is the soft tuft loaded with ink. That is the whole word, the implement drawn in two materials. The traditional 筆 keeps the bamboo but puts a hand on the brush (聿) instead of the hair, the same object from the side of the one who holds it.

From the brush in the hand, follow the ink. Touch it to paper and you get a stroke (笔画); let the hand run and you get writing and style (笔记, 文笔); enter the result in a ledger and each line becomes a count (一笔钱, 一笔生意). Wherever you meet 笔, picture the loaded tip touching the page, and ask whether it is the tool, the writing, or the entry in the book.