The Collaborative International Dictionary of English v.0.48:

Quidam \Qui"dam\, n. [L.]
   Somebody; one unknown. --Spenser.
   [1913 Webster]

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

QUIDAM, French law. Some, one; somebody. This Latin word is used to express
an unknown person, or one who cannot be named.
     2. A quidam is usually described by the features of his face, the color
of his hair, his height, his clothing, and the like in any process which may
be issued against him. Merl. Repert. h.t.; Encyclopedie, h.t.
     3. A warrant directing the officer to arrest the "associates" of
persons named, without naming them, is void. 3 Munf. 458.