Thanks MT - my apologies for asking the question and then not jumping back in. What you say makes sense - appreciate the reply.Good question because the dictionary definition doesn't really capture how the term is used.
A shortcut to thinking about populism is that it's a rebellion against elitism (also pretty vaguely defined). You know how people complain about "the elites" such as globalist free-traders, deep-state public servants, out-of-touch journalists, ivory-tower professors, self-serving institutional leaders, and know-it-all scientists with all of their technocratic, evidence-based solutions published in fancy-sounding journals and whatnot? Populists are the ones attacking all that stuff whether from the left ("we need protectionist tariffs to protect the common folk in this country") or from the right ("we need a border wall to protect the common folk in this country").
Populists distrust professional expertise, preferring simple, common-sense slogans filled with typos. A populist politician focuses on issues where a man on the street would most sharply disagree with a university professor in his chosen field -- about trade policy, immigration, whatever -- and sides with the man on the street.
Populism is correlated with authoritarianism because it takes a strong leader, who must be given extraordinary powers, to stand up to all of the various institutions run by elites -- i.e., to drain the swamp.