1to irrigate
2to pour water into
3to inject water into meat to increase its weight (plumping)
4to cook the books
5to post low-value messages (small talk etc) on Internet forums
Example
nǐ你zěn me怎么yì一yǒu kòng有空jiù就shàng wǎng上网，chōng làng冲浪guàn shuǐ灌水dǎ打QQQQ，yí ge一个yè wǎn夜晚jiù就rēng扔gěi给tā它le了？
你怎么一有空就上网，冲浪灌水打QQ，一个夜晚就扔给它了？
Why do you go online, surf, chat, and use QQ every time you have free time, wasting an entire night on it?