"It's worth noting that Forbes caught quite a bit of flack because its article posted numerous incorrect population figures for the various cities when compared with the latest Census counts."
Old-time copy editors insisted that flak was the word that means "criticism; hostile reaction." Flak originally was World War II slang for anti-aircraft fire, derived, according to Paul Dickson's "War Slang," from "the German Fliegerabwehrkanone (a gun used to drive off aircraft)." By extension, flak came to mean verbal enemy fire too.
Random House more or less agrees with Dickson on the origin of flak, but it also says that flack is now an acceptable alternate spelling. Flack originally was a "sometimes disparaging" slang term for a publicist, a PR man. The same copy editors who insisted on the flak spelling for "abuse" were scornful of flacks, largely because the flacks made more money than the editors. Random House says that flack dates from 1935-40 — making it the same age as flak — and is "said to be after Gene Flack, a movie publicity agent."
A news magazine reported that an actor slapped a Ukrainian journalist who'd tried to kiss him and then said "He's lucky I didn't sucker punch him." A drama review in the same issue said: "High-concept versions of Shakespeare don't always succeed, but once this one gets rolling, you'll feel it like a sucker punch."
As every storm became a firestorm, and every start a jump start, it now seems that every punch is becoming a sucker punch. That term used to be applied exclusively to a sneak attack, a punch thrown while the other party was unsuspecting, maybe even reaching out to shake hands. Now, even a punch thrown in heated battle might be called a sucker punch, and the term seems to refer to severity as much as surprise.
The on-line Urban Dictionary says that a sucker punch "primarily involves a closed fist contacting the soft underbelly of a person (beneath the rib cage) at a high velocity, causing the ensuing force to press upward on the victim's diaphragm, leading to a sudden expulsion of air from the victim's mouth and lungs. This opening blow leaves the victim open to various other attacks ..."