The Collaborative International Dictionary of English v.0.48:

Quinquagesima \Quin`qua*ges"i*ma\, a. [L., fr. quinquagesimus
   the fiftieth, akin to quinquaginta fifty, quinque five. See
   Five.]
   Fiftieth.
   [1913 Webster]

   Quinquagesima Sunday, the Sunday which is the fiftieth day
      before Easter, both days being included in the reckoning;
      -- called also Shrove Sunday.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Quinquagesima Sunday
    n 1: the Sunday before Ash Wednesday (the beginning of Lent)
         [syn: Quinquagesima, Quinquagesima Sunday]