It is always important when trying to work out the meaning of a line or a group of lines to look at them in context of the rest of the scene or chapter. Let us remember that Lucius, a servant of Brutus, enters telling him that Cassius is at the door with others. When Brutus asks whom these "others" are and if he knows them, note how Lucius responds:
No, sir; their hats are plucked about their ears,
And half their faces buried in their cloaks,
That by no means I may discover them
By any mark of favour.
Thus the quote "their hats are plucked about their ears" relates to the way that the conspirators, for this is obviously who is with Cassius, are trying to hide their identity so that they cannot be recognised. Just as they are burying their faces in their cloaks, so they have their hats so firmly on their head so that their faces are concealed and they may not be recognised.
The Modern translation would be : No, sir, their hats are pulled down over their ears and their faces are half buried under their cloaks, so there's no way to tell who they are.