It's intended to imply two equally false claims: (1) a refusal to march enthusiastically behind the Wall-Street-enriched, multiple-war-advocating, despot-embracing Hillary Clinton is explainable not by ideology or political conviction, but largely if not exclusively by sexism: demonstrated by the fact that men, not women, support Sanders (his supporters are "bros"); and (2) Sanders supporters are uniquely abusive and misogynistic in their online behavior. Needless to say, a crucial tactical prong of this innuendo is that any attempt to refute it is itself proof of insensitivity to sexism if not sexism itself (as the accusatory reactions to this article will instantly illustrate).

It's become such an all-purpose, handy pro-Clinton smear that even consummate, actual "bros" for which the term was originally coined – straight guys who act with entitlement and aggression, such as Paul Krugman  – are now reflexively (and unironically) applying it to anyone who speaks ill of Hillary Clinton, even when they know nothing else about the people they're smearing, including their gender, age or sexual orientation.