The response to our advertisement has been very disappointing.

disappointing 是形容词，由动词 disappoint（使失望）加上后缀 -ing 构成。dis- 是表示否定的前缀，appoint 原意为"任命、指定"，所以 disappoint 字面意思是"使...的期望落空"，即"使失望"。disappointing 表示"令人失望的"。