noun
illegible handwriting; hard-to-read characters
Refers to handwriting that is difficult to decipher due to personal quirks or poor penmanship.
彼の癖字は本当に読みにくい。
His handwriting is really hard to read.
この手紙は癖字が多くて、解読に時間がかかった。
This letter had a lot of illegible handwriting, so it took time to decipher.
悪筆 refers to objectively bad or ugly handwriting, while 癖字 emphasizes the idiosyncratic, hard-to-read nature of the writing.
達筆 means skillful or flowing handwriting, the opposite of 癖字.
Compound of 癖 (くせ, 'habit, quirk') and 字 (じ, 'character, handwriting').