a face with a smile to show people that the person is happy for them, but without having true emotions.

In the Big Bang Theory TV series, Sheldon’s koala face is the smile that forms on his face when he’s watching koalas eat eucalyptus at the zoo. He uses the same face to show people (his girlfriend Amy in specific) that he is happy for them.
Sheldon: Thank you. Amy, I’d like to apologize. Your accomplishment was impressive. And I’m proud of you.

Amy: We both know that’s your koala face.
by Ciphers November 7, 2013
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When a koala attacks and rips your face off and throws it on the ground with disdain.
Frequently misused as 'koala your ass' which actually means that a koala has ripped your ass off.
Eric got all honeyed up for Geoff's cash room but discovered that there was no cash to be found, so he said to Geoff, 'I'm going to koala your face'...and he did.
by wealthy in forest heights November 28, 2008
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