The Collaborative International Dictionary of English v.0.48:

Misjoinder \Mis*join"der\, n. (Law)
   An incorrect union of parties or of causes of action in a
   procedure, criminal or civil. --Wharton.
   [1913 Webster]

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

MISJOINDER, pleading. Misjoinder of causes of action, or counts, consists in
joining, in different counts in one declaration, several demands, which the
law does not permit to be joined, to enforce several distinct, substantive
rights of recovery; as, where a declaration joins a count in trespass with
another in case, for distinct wrongs or a count in tort, with another in
contract. Gould. 6n PI. c. 4, Sec. 98; Archb. Civ. PI. 61, 78 176; Serg. and
Rawle, 358; Dane's Ab. Index, h.t.
     2. Misjoinder of parties, consists in joining as plaintiffs or
defendants, persons, who have not a joint interest. When the misjoinder
relates to the plaintiffs, the defendants may, at common law, plead the
matter in abatement, whether the action be real; 12 H. IV., 15; personal;
Johns. Ch. R. 350, 438; 12 John. R. 1; 2 Mass. R. 293; or mixed; or it will
be good cause of nonsuit at the trial. 3 Bos. & Pull. 235. Where the
objection appears upon the face of the declaration, the defendant may demur
generally; 2 Saund. 145; or move in arrest of judgment; or bring a writ of
error.
     3. When in actions ex contractu against several, there is a misjoinder
of the defendants, as if there be too many persons made defendants, and the
objection appears on the pleadings, either of the defendants may demur, move
in arrest of judgment, or support a writ of error; and, if the objection do
not appear on the pleadings, the plaintiff may be nonsuited upon the trial,
if he fail in proving a joint contract. 5 Johns. R. 280; 2 Johns. R. 213; 11
Johns. R. 101; 5 Mass. R. 270.
     4. In actions ex delicto, the misjoinder cannot in general be objected
to, because in actions for torts, one defendant may be found guilty and the
others acquitted. Archb. Civ. Pl. 79. As to the cases in which a misjoinder
may be aided by a nolle prosequi, see 2 Archb. Pr. 218-220.