古 is a compound ideograph. 古 = 十 + 口. 十 means countless in ancient Chinese; 口 is a mouth. 古 depicts that ancient stories were told by countless people and passed down through generations, so the original meaning of 古 is “ancient/old”.
古 serves as a phonetic component to form characters.
Characters formed by the phonetic component 古
苦、 胡、 姑、 故、 枯、 固、 估、 克、 咕、 辜、 牯、 罟、 蛄、 嫴、 沽、 钴、 狜、 鸪、 鮕、 怘、 秙、 跍、 岵、 嘏、 呄、 酤、 怙、 骷、 祜、 诂、 轱