= Don't Contradict your parents. (or other elders)
ex. Don't contradict me like that, you little bastard!
It should depends on the context, well it always be used when you are buying something which is sold by weight, for example, fruit or vegetable, and the price is very expensive, so before you take it to check out, you weight it again and again, wondering if it is too heavy to pay...
ex. the dealer: stop weighing it ! buy it or leave it!
= Just at the right time. (just=而已)
"更是" emphasize "少之又少"
"少之又少" emphasize " 少"
ex. 會漢語的人已經很少了, 會廣東話的人更是少之又少.
There is few ppl who can speak Mandarin, however,
there is fewer ppl who can speak Contonese.