The Collaborative International Dictionary of English v.0.48:

Assignor \As`sign*or"\, n. [L. assignator. Cf. Assigner.]
   (Law)
   An assigner; a person who assigns or transfers an interest;
   as, the assignor of a debt or other chose in action.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assignor
    n 1: (law) the party who makes an assignment

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

ASSIGNOR. One who makes an assignment; one who transfers property to
another.
     2. In general the assignor can limit the operation of his assignment,
and impose whatever condition he may think proper, but when he makes a
general assignment in trust for the use of his creditors, he can impose no
condition whatever which will deprive them of any right; 14 Pick. 123; 15
John. 151; 7 Cowen, 735; 5 Cowen, 547 20 John. 442; 2 Pick. 129; nor any
condition forbidden by law; as giving preference when the law forbids it.
     3. Ad assignor may legally choose his own trustees. 1 Binn. 514.