character of questionable form
A technical term used in textual criticism, paleography, or character studies to refer to a written character whose shape, authenticity, or correctness is uncertain. Not a common everyday word.
This manuscript has many characters of questionable form, making it difficult to decipher.
Compound of 疑 (doubt, suspicion) and 字 (character, letter). The exact historical origin is uncertain, but it follows a common pattern for technical terms in Japanese philology.