What does 'fall flat' mean?

[Ondly]

English Language Tests, Intermediate level

ESL/EFL Test #465 "English Slang Idioms (207)", question 4

"I told a joke in my presentation and it fell .......... No one laughed and I got dirty looks from several of the people there. I thought it was funny and I think they all need to develop a sense of humor," Steven said to Lewis.

(a) round
(b) flat
(c) on
(d) back

English Language Tests, Intermediate level

ESL/EFL Test #465 "English Slang Idioms (207)", answer 4

"I told a joke in my presentation and it fell flat. No one laughed and I got dirty looks from several of the people there. I thought it was funny and I think they all need to develop a sense of humor," Steven said to Lewis.

Correct answer: (b) flat
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fell flat is an idiom i want to know the meaning of this only briefly.
thanks

[Mister Micawber]

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In that case, you might try a DICTIONARY.
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Native English teacher at Mister Micawber's