Lost the thread of your argument

[Izik]

English Language Tests, Intermediate level

ESL/EFL Test #62 "Losing It (2)", question 6

Do you mind repeating that last bit again as I seem to have lost the ......... of your argument?

(a) length
(b) string
(c) cord
(d) thread

English Language Tests, Intermediate level

ESL/EFL Test #62 "Losing It (2)", answer 6

Do you mind repeating that last bit again as I seem to have lost the thread of your argument?

Correct answer: (d) thread

Your answer was: correct
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what does the phrase 'lost the thread of your argument' mean??

[Alan]