The Collaborative International Dictionary of English v.0.48:

Nudum pactum \Nu"dum pac"tum\ [L., a nude pact.] (Law)
   A bare, naked contract, without any consideration and
   therefore unenforceable in a court of law. --Tomlins.
   [1913 Webster +PJC]

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

NUDUM PACTUM, contracts. A contract made without a consideration,; it is
called a nude or naked contract, because it is not clothed with the
consideration required by law, in order to give an action. 3 McLean, 330; 2
Denio, 403; 6 Iredell, 480; 1 Strobh. 329; 1 Kelly, 294; 1 Dougl. Mich. R.
188.
     2. There are some contracts which, in consequence of their forms,
import a consideration, as sealed instruments, and bills of exchange, and
promissory notes, which are generally good although no consideration
appears.
     3. A nudum pactum may be avoided, and is not binding.
     4. Whether the agreement be verbal or in writing, it is still a nude
pact. This has been decided in England, 7 T. R. 350, note; 7 Bro. P. C. 550;
and in this country; 4 John. R. 235; 5 Mass. R. 301, 392; 2 Day's R. 22. But
if the contract be under seal, it is valid. 2 B. & A. 551. It is a rule that
no action can be maintained on a naked contract; ex nudo pacto non oritur
actio: 2 Bl. Com. 445; 16 Vin. Ab. 16.
     5. This term is borrowed from the civil law, and the rule which decides
upon the nullity of its effects, yet the common law has not; in any degree
been influenced by the notions of the civil law, in defining what
constitutes a nudum pactum. Dig. 19, 5, 5. See on this subject a learned
note in Fonb. Eq. 335, and 2 Kent, Com. 364. Toullier defines nudum pactum
to be an agreement not executed by one of the parties, tom. 6, n. 13, page
10. Vide 16 Vin. Ab. 16; 1 Supp. to Ves. jr. 514; 3 Kent, Com. 364; 1 it.
Pr. 113; 8 Ala. 131; and art. Consideration.