What does phrase mean: "very hot under the collar"?

[Micha]

English Idioms and Expressions, Advanced Level

ESL/EFL Test #19 "Hot expressions", question 9

I thought he was going to burst with anger because he was getting very hot under the collar.

(a) very silly
(b) very happy
(c) very angry
(d) very loud

English Idioms and Expressions, Advanced Level

ESL/EFL Test #19 "Hot expressions", answer 9

I thought he was going to burst with anger because he was getting very angry.

Correct answer: (c) very angry
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meaning of very hot under the collar?

Micha

[Alan]

meaning: very angry.

Alan
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English as a Second Language
You can read my ESL story Too Many Words

[Conchita]

If you look at the test answer, you will see that it means 'very angry'. This idiom has also the sense of 'embarrassed'.