'Wack' is an adjective and it is used especially in America to mean either 'very bad; not of good quality' or 'very strange'. It is used to mean 'very strange' here. The man's behaviour or action seemed to be very strange since he left the meeting early. 'Wacky' (also 'whacky') is an informal word which means funny or amusing in a slightly crazy way. It is not a synonym for 'wack' as Peachy said.

Here it means that the move/decision was unwise.

it means an eccentric, an oddball ,a weirdo cowboy...

Wack = wacky (crazy). It's more a US term. We have a different meaning in Australia - we say something is "out of whack" to mean it isn't working correctly.

We hear it in American films, and know what it means, but we don't tend to use that word in the UK.