Green’s Dictionary of Slang

flak n.

also flack
[SE flak, anti-aircraft fire, ult. the initials of the elements of Ger. fliegerabwehrkanone ‘pilot-defence-gun’]
(orig. US)

1. [1930s+] (also flak-artist, flak merchant) a publicity man/woman, a press agent [parallel ety. suggests Gene Flack, a 1930s US film publicist].

2. [1940s] interference, annoyance, problems.

3. [1960s+] cheek, negative criticism, verbal attacks.

4. [1970s+] publicity material.

In compounds

flak catcher (n.)

[1970s+] a civil servant or similar figure in private industry whose task is to intercept complaints, queries etc. from the public, before such problems reach their superiors.

In phrases

catch flak (v.) (also catch flack, get flack/flak)

[1960s+] (US) to receive criticism, to face verbal attacks.