What is a 'left-handed invitation'?

English as a Second Language English Vocabulary, Grammar and Idioms
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What exactly is a ‘left-handed invitation’?[YSaerTTEW443543]

TOEFL listening lectures: A university lecture in Biology[YSaerTTEW443543]

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Without any further context, I’d say that using ‘left-handed’ puts the sincerity of the invitation in doubt.
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Makes sense, Amy. Thanks a lot for responding so fast.[YSaerTTEW443543]

TOEFL listening lectures: Why are red crosses erected on Tanna Island?[YSaerTTEW443543]