[ http://en.wiktionary.org/wiki/%E4%BA%91#Mandarin ] says it is literary use. [ http://www.xiaoma.info/hanzi.php?hz=%E4%BA%91&fhz=%E4%BA%91 ] says it's classical. From [ http://www.nciku.com/search/zh/detail/%E4%BA%91/1318867 ]: 小张一直都是人云亦云，根本就是个应声虫。 Xiaozhang is always repeating what others say, a complete yes man.

meant 'cloud’ is a modern Chinese word. meant 'say/speak' is an ancient Chinese word,and it is no longer used.For example:“古语有云”means 'as the old saying said...'.But it is an idiom“人云亦云”used now,it means you say whatever people say.

云 : 《說文》山川氣也。象回轉形。後人加雨作雲，而以云爲云曰之云。 《正字通》與曰音別義同。凡經史，曰通作云。 清代段玉裁『說文解字注』 古文省雨。古文上無雨。非省也。二葢上字。象自下回轉而上也。正月。昏姻孔云。傳曰。云，旋也。此其引伸之義也。古多叚云爲曰。如詩云卽詩曰是也。亦叚員爲云。如景員維河箋云員古文作云，昏姻孔云本又作員，聊樂我員本亦作云，尙書云來衞包以前作員來，小篆妘字籒文作?是。云員古通用。皆叚借風雲字耳。自小篆別爲雲而二形迥判矣。 云 本來是 cloud 的象形 後來添加了 雨 rain 使成 雲 而 云 就作 say / speak 用 很早很早以前這二字 就已經分化了 各自承擔不同的意義 (自小篆別爲雲而二形迥判矣。) 哪知 中共簡化漢字 又將他們混同了

yeah,云 in modern Chinese means Cloud, but in ancient China it means say/speak and cloud.

汉语学的不错啊！继续努力。