A molossus is a metrical foot made up of three long syllables in classical quantitative verse, or, in stress-based poetry, three strongly stressed syllables in a row. It is uncommon in English verse, but poets may use it for emphasis because the heavy, forceful rhythm can slow the line and make a phrase sound blunt or emphatic.
Example
"Stand firm now, all!"
The three stressed beats give the phrase a heavy, forceful push.