The Collaborative International Dictionary of English v.0.48:

Constable \Con"sta*ble\ (k[o^]n"st[.a]*b'l or
   k[u^]n"st[.a]*b'l), n. [OE. conestable, constable, a
   constable (in sense 1), OF. conestable, F. conn['e]table, LL.
   conestabulus, constabularius, comes stabuli, orig., count of
   the stable, master of the horse, equerry; comes count (L.
   companion) + L. stabulum stable. See Count a nobleman, and
   Stable.]
   1. A high officer in the monarchical establishments of the
      Middle Ages.
      [1913 Webster]

   Note: The constable of France was the first officer of the
         crown, and had the chief command of the army. It was
         also his duty to regulate all matters of chivalry. The
         office was suppressed in 1627. The constable, or lord
         high constable, of England, was one of the highest
         officers of the crown, commander in chief of the
         forces, and keeper of the peace of the nation. He also
         had judicial cognizance of many important matters. The
         office was as early as the Conquest, but has been
         disused (except on great and solemn occasions), since
         the attainder of Stafford, duke of Buckingham, in the
         reign of Henry VIII.
         [1913 Webster]

   2. (Law) An officer of the peace having power as a
      conservator of the public peace, and bound to execute the
      warrants of judicial officers. --Bouvier.
      [1913 Webster]

   Note: In England, at the present time, the constable is a
         conservator of the peace within his district, and is
         also charged by various statutes with other duties,
         such as serving summons, precepts, warrants, etc. In
         the United States, constables are town or city officers
         of the peace, with powers similar to those of the
         constables of England. In addition to their duties as
         conservators of the peace, they are invested with
         others by statute, such as to execute civil as well as
         criminal process in certain cases, to attend courts,
         keep juries, etc. In some cities, there are officers
         called high constables, who act as chiefs of the
         constabulary or police force. In other cities the title
         of constable, as well as the office, is merged in that
         of the police officer.
         [1913 Webster]

   High constable, a constable having certain duties and
      powers within a hundred. [Eng.]

   Petty constable, a conservator of the peace within a parish
      or tithing; a tithingman. [Eng.]

   Special constable, a person appointed to act as constable
      of special occasions.

   To overrun the constable, or outrun the constable, to
      spend more than one's income; to get into debt. [Colloq.]
      --Smollett.
      [1913 Webster]

WordNet (r) 3.0 (2006):

constable
    n 1: a lawman with less authority and jurisdiction than a
         sheriff
    2: English landscape painter (1776-1837) [syn: Constable,
       John Constable]
    3: a police officer of the lowest rank [syn: constable,
       police constable]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "constable":
   G-man, John Law, MP, bailiff, beadle, beagle, bobby, bound bailiff,
   bull, captain, catchpole, chief of police, commissioner, cop,
   copper, deputy, deputy sheriff, detective, fed, federal, flatfoot,
   flic, fuzz, gendarme, government man, inspector, lictor,
   lieutenant, mace-bearer, marshal, mounted policeman, narc, officer,
   paddy, patrolman, peace officer, peeler, police captain,
   police commissioner, police constable, police inspector,
   police matron, police officer, police sergeant, policeman,
   policewoman, portreeve, reeve, roundsman, sergeant,
   sergeant at arms, sheriff, superintendent, tipstaff, tipstaves,
   trooper



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

CONSTABLE. An officer, who is generally elected by the people.
     2. He possess power, virture officii, as a conservator of the peace at
common law, and by virtue of various legislative enactments; he. way
therefore apprehend a supposed offender without a warrant, as treason,
felony, breach of the peace, and for some misdemeanors less than felony,
when committed in his view. 1 Hale, 587; 1 East, P. C. 303 8 Serg. & Rawle,
47. He may also arrest a supposed offender upon the information of others
but he does so at his peril, unless he can show that a felony has been
committed by some person, as well as the reasonableness of the suspicion
that the party arrested is guilty. 1 Chit. Cr. L. 27; 6 Binn. R. 316; 2
Hale, 91, 92 1 East, P. C. 301. He has power to call others to his
assistance; or he may appoint a deputy to do ministerial acts. 3 Burr. Rep.
1262.
     3. A constable is also a ministerial officer, bound to obey the
warrants and precepts of justices, coroners, and sheriffs. Constables are
also in some states bound to execute the warrants and process of justices of
the peace in civil cases.
     4. In England, they have many officers, with more or less power, who
bear the name of constables; as, lord high constable of England, high
constable 3 Burr. 1262 head constables, petty constables, constables of
castles, constables of the tower, constables of the fees, constable of the
exchequer, constable of the staple, &c.
     5. In some of the cities of the United States there are officers who
are called high constables, who are the principal police officers where they
reside. Vide the various Digests of American Law, h.t.; 1 Chit. Cr. L. 20;
5 Vin. Ab. 427; 2 Phil. Ev. 253 2 Sell. Pr. 70; Bac. Ab. h.t.; Com. Dig.
Justices of the Peace, B 79; Id. D 7; Id, Officer, E 2; Wille. Off. Const.