“Flak” is WW II airman’s slang for shells being fired at you in the air, so to catch a lot of flak is to feel in danger of being shot down. However, most civilians these days have never heard of “flak,” so they use “flack” instead, which originally meant “salesman” or “huckster.” You need to worry about this only if you’re among old-time veterans.
You’re more likely to embarrass yourself if you mix up the expression “catch a lot of flak” with “give a lot of slack,” which has almost the opposite meaning. You can’t catch slack.
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