It definitely sounds silly in English, but it's still idiomatic Portuguese today ("um surdo").
My hypothesis about this is that Portuguese has preserved some of the Latin tendency to allow arbitrary adjectives to be used as substantives (nouns). In Latin you can do this with any adjective in any context to refer to a person or thing with the specified characteristic (so "laetus" 'happy' could also mean 'a happy person', "tristis" 'sad' could also mean 'a sad person'; compare the motto of Kansas, "ad astra per aspera", which uses the adjective "aspera" as a substantive meaning 'difficult (things)').
In Portuguese you can find many adjectives that can be used in an identical form as substantives -- certainly not always, but seemingly more often than in English.
No shame there, I'm sure you speak English far better than I do whichever language is native to you. Now, if you decided to publish a phrase book based on your current knowledge, then we'd be having a bit of a laugh.
It’s a common error among English learners, particularly when it comes to nationalities. We would say “I am French” or “I am a French person”, but never “I am a French”.
Made even more confusing that you can do it with some nationalities but not others. So "a French" is out, but most nationalities ending "-an" are OK: an American, a German, an Afghan, a Norwegian, a Russian, an Italian. Also "-ese", though with something of an archaic air: a Chinese, a Maltese, a Portugese. Also a Swiss, an Argentine, an Israeli, because why not?
And of course pretty much any adjective can be used as a plural noun with the definite article: the French, the Dutch, the beautiful and the damned, the quick and the dead, etc.
What's more, some nationalities have specific nouns for them, but not all. A Spanish person is a Spaniard, a Finnish person is a Finn, but a German person isn't a Germ.
Then there are those that fell out of use. There are Englishmen, Irishmen, Frenchmen, but never Chinamen!
"Oh I say there Bart, is that a deaf I see? Why bless the fat of my leg!"