Europe. It usually means when one tries to grab votes at the cost of doing what's "correct" from one's point of view. A stereotypical case would be providing "bread and circus" in the short term while the country is sinking in the mid term.

The following headlines are example use of "populist" with negative connotations (they are not the only ones, I just provide anecdotal evidence of the common use of "populist" as a negative trait).

British English:

http://www.bbc.co.uk/news/world-europe-17837098

http://news.bbc.co.uk/democracylive/hi/house_of_lords/newsid... (beware starts parliamentary video automatically)

http://www.guardian.co.uk/commentisfree/2011/aug/23/punitive...

French:

http://www.lemonde.fr/idees/ensemble/2012/02/09/le-populisme...

Spanish:

http://www.abc.es/20120515/opinion/abcp-populista-eres-20120...