What does 'killjoy' mean?

[d]

Test No. incompl/elem-17 "White Water Rafting", question 8

Oh, come on. Why do you have to be such a kill ..........

(a) joy
(b) joke
(c) fun

Test No. incompl/elem-17 "White Water Rafting", answer 8

Oh, come on. Why do you have to be such a kill joy.

Correct answer: (a) joy
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what?s joy

[Torsten]

Joy means pleasure or happiness and a kill joy (or killjoy) is a person who spoils the fun or enjoyment of other people.

TOEIC listening, photographs: View on the harbour

[Rich7]

Also called "downer"(American E.) or buzzkill.