Idiom "old hat"

[Spacetoon]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #41 "Talking through his hat", question 8

If you want to keep up with the latest fashion, you shouldn't wear that -- it's ......... hat now.

(a) late
(b) old
(c) finished
(d) gone

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #41 "Talking through his hat", answer 8

If you want to keep up with the latest fashion, you shouldn't wear that -- it's old hat now.

Correct answer: (b) old
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It is old hat. I don't get it, isn't supposed to be AN OLD HAT ? I mean to put an as an article ?

Thanks in advance.

Spacetoon

[Pamela]

No, the idiom old hat goes with a zero article and means old fashioned in the test sentence.

[Spacetoon]

Well, I see now, it is an idiom after all..Thanks Pamela.