His handwriting was messy, but the note was still decipherable.

decipherable 是由动词 decipher（破译、辨认）加上形容词后缀 -able 构成的形容词。decipher 本身由前缀 de-（表示"去除"）和 cipher（密码）组成，字面意思是"解密"。 -able 后缀表示"能够...的"，所以 decipherable 意思是"能够被辨认出来的"。