What is a 'left-handed invitation'?

[Torsten]

What exactly is a 'left-handed invitation'?

TOEFL listening lectures: A university lecture in Biology

[Yankee]

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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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"Nearly all men can stand adversity, but if you want to test a man's character, give him power." ~ Abraham Lincoln

[Torsten]

Makes sense, Amy. Thanks a lot for responding so fast.

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