The Collaborative International Dictionary of English v.0.48:

Jointress \Joint"ress\, n. (Law)
   A woman who has a jointure. [Written also jointuress.]
   --Blackstone.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

JOINTRESS or JOINTURESS. A woman who has an estate settled on her by her
husband, to hold during her life, if she survive him. Co. Litt. 46.