Deadline (9)
(Other definitions for ultimatum that I've seen before include "Final warning" , "Final statement of actions required of another" , "A final demand - or else" , "Threat" , "Final offer" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deadline</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for time limit that I've seen before include "Point before which something must be achieved" , "Deadline" , "within which task must be done" , "Prescribed term" , "maximum duration" .)
(Other definitions for et tu brute that I've seen before include "Shakespeare's version of Julius Caesar's last words, in Latin" , "Caesar's reported words on being stabbed" , "Julius Caesar's supposed last words" , "According to the Bard, Caesar's final words" , "Julius Caesar's last words to unexpected assassin" .)