A word that is particularly offensive or controversial and thus sparks an immediate emotional reaction. The phrase may reference the red flag because it is the tool traditionally used by matadors to aggravate bulls. How dare you say a red-flag term like that to me, your own mother! Please don't say any red-flag terms at this dinner party.
See also: term
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.