灌水 translates to 'irrigation' in English.

You better provide us with the context to determine the precise meaning ,because as mentioned above it has more than one meaning.

灌水 = lit. to irrigate / to artificially increase weight / to cook the books / to leave a tasteless message

中文里的“灌水”一词形象生动，一些人为了获得积分在论坛里反复留言，在回别人帖子的时候没有做出交际性的评论，只是简单的表示“同意”、“支持”，内容与主题无关，这种现象在英语中叫 bump，它在论坛里是“顶”的意思：To bump a thread on an internet forum is to post a reply in order to raise the thread's profile by returning it to the top of the list of active threads. This is also called "necroposting". 这种“灌水”往往被认为是一种垃圾留言（spam），被很多论坛禁止。