網上字典標示某些words 是informal words

Informal words mostly pertaining to those words are 口語化 or slang. That means you can use it verbally but not to use in formal correspondences or compositions/essays. For example, when people ask if they can do something, in correspondences, you can say go ahead. Do not use "shoot" when writing. You can use it when you talk to somebody to ask them to approve your action though.

用不用 informal words 看對象和場合
editorial 用 informal words, 因每個人看得明, 並且一針見血
太 formal, 給人感覺是咬文嚼字, pedantic (賣弄學問的), 缺乏真誠
考試寫文, 官方公告, 公司書信來往, 用 formal English
寫故事內容中交談, 或者對象是普羅大眾 (proletariat), 用 informal English
近年, 鼓吹用 plain English (not informal), 適合 debate, job interview, editorial

2014-01-26 09:49:57 補充：
如同中文 "唔好做飛仔飛女"
"請勿變成不良少年"

It is nice to know you.
It has been a pleasure to meet you.
It is a privilege to become acquainted with you.

Your coat is dirt cheap.
I am afraid your coat is relatively inexpensive.