noun
poor writing; messy handwriting and composition
A humble, self-deprecating phrase used to apologize for one's own messy handwriting and disorganized writing, often in letters or notes.
乱筆乱文で失礼します。
I apologize for my messy handwriting and poor writing.
乱筆乱文をお許しください。
Please forgive my scribbling.
悪文 means poorly written prose in general, without the humble, self-deprecating nuance of 乱筆乱文.
Compound of 乱筆 (messy handwriting) and 乱文 (disorganized writing), both using 乱 (disorderly). The exact historical origin is uncertain, but it is a conventional set phrase.