verb/sɑ̃.tiʁ lə sufʁ/
to appear volatile, tense, or dangerous
“Mais rien n'était écrit dans ce quart de finale qui sentait le soufre entre les vieux rivaux.” — But everything was open in this very tense quarter final between old rivals.
Literally, “to smell of sulfur”, perhaps alluding to explosives (sulfur being one of the ingredients of black powder). Alternatively, from biblical connotations of hell, as in brimstone.