What does it mean 'words just fail me'?

[Luby]

Business Idiom in English, Intermediate level

ESL/EFL Test #132 "Ways of showing pleasure", question 4

Words just ......... me when I try to say how happy I am.

(a) fail
(b) miss
(c) pass
(d) fool

Business Idiom in English, Intermediate level

ESL/EFL Test #132 "Ways of showing pleasure", answer 4

Words just fail me when I try to say how happy I am.

Correct answer: (a) fail

Your answer was: incorrect
Words just pass me when I try to say how happy I am.
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Good Evening!
What does it mean 'words just fail me'??

[Beeesneees]

"I am unable to find the words I need (to express how I feel)."
Words have 'failed' the writer because s/he cannot find any words which accurately convey the mood.
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Cheers m' dears!