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