This is the 9662nd most frequent Japanese word.
自筆
"自筆" means "one's own handwriting."
The word '自筆' here refers to something written personally in one's own handwriting.
彼は自筆で詩を書いた。
He wrote a poem in his own handwriting.
'自筆' is used to indicate the author handwritten the letter themselves.
その手紙は彼の自筆によるものです。
That letter is written by him personally.
Here, '自筆' refers specifically to the handwriting being one's own, often used in formal confirmation scenarios.