Hi Madina It comes from the noun 'common sense', which means a good decision or action based on knowledge which we learn from experience of the world or instinctively know - as opposed to having studied in school or college or read in a book. So the phrase 'Use a bit of common' is used by the speaker to suggest that the listener needs to not do something which, in the speaker's opinion, is obviously stupid. Example: "You're going out in the snow without a coat? Use a bit of common!" (in other words 'Put a coat on. It is cold.') It's slang, so not to be used in formal situations. Hope this helps. Chris

I've never heard this. Of course, "You must use a bit of common sense" is an old and well known phrase. Maybe the shortened form is British English.

'Use a bit of common sense' is used if somebody is doing or saying something that seems to the speaker to be obviously silly or dangerous. It means 'use the basic knowledge of the world and simple intelligence that is common to every human being'

use a bit of common sense. (sound practical judgment that is independent of specialized knowledge, and training)