The Collaborative International Dictionary of English v.0.48:

Mourn \Mourn\, v. t.
   1. To grieve for; to lament; to deplore; to bemoan; to
      bewail.
      [1913 Webster]

            As if he mourned his rival's ill success. --Addison.
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            And looking over the hills, I mourn
            The darling who shall not return.     --Emerson.
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   2. To utter in a mournful manner or voice.
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            The lovelorn nightingale
            Nightly to thee her sad song mourneth well.
                                                  --Milton.
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   Syn: See Deplore.
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The Collaborative International Dictionary of English v.0.48:

Mourn \Mourn\ (m[=o]rn), v. i. [imp. & p. p. Mourned
   (m[=o]rnd); p. pr. & vb. n. Mourning.] [AS. murnan; akin to
   OS. mornian, OHG. mornen, Goth. ma['u]rnan.]
   1. To express or to feel grief or sorrow; to grieve; to be
      sorrowful; to lament; to be in a state of grief or
      sadness.
      [1913 Webster]

            Abraham came to mourn for Sarah, and to weep for
            her.                                  --Gen. xxiii.
                                                  2.
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   2. To wear the customary garb of a mourner.
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            We mourn in black; why mourn we not in blood?
                                                  --Shak.
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            Grieve for an hour, perhaps, then mourn a year.
                                                  --Pope.
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WordNet (r) 3.0 (2006):

mourn
    v 1: feel sadness; "She is mourning her dead child"
    2: observe the customs of mourning after the death of a loved
       one

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "mourn":
   ache, agonize, bemoan, bewail, bleed, brood over, deplore, dirge,
   discomfort, dismay, distress, elegize, fret, give sorrow words,
   grieve, grieve over, keen, knell, lament, moan, mope, pain, pine,
   pine away, regret, repine, rue, sigh, sing the blues, sorrow,
   take on, weep for, weep over



Easton's 1897 Bible Dictionary:

Mourn
   Frequent references are found in Scripture to, (1.) Mourning for
   the dead. Abraham mourned for Sarah (Gen. 23:2); Jacob for
   Joseph (37:34, 35); the Egyptians for Jacob (50:3-10); Israel
   for Aaron (Num. 20:29), for Moses (Deut. 34:8), and for Samuel
   (1 Sam. 25:1); David for Abner (2 Sam. 3:31, 35); Mary and
   Martha for Lazarus (John 11); devout men for Stephen (Acts 8:2),
   etc.

     (2.) For calamities, Job (1:20, 21; 2:8); Israel (Ex. 33:4);
   the Ninevites (Jonah 3:5); Israel, when defeated by Benjamin
   (Judg. 20:26), etc.

     (3.) Penitential mourning, by the Israelites on the day of
   atonement (Lev. 23:27; Acts 27:9); under Samuel's ministry (1
   Sam. 7:6); predicted in Zechariah (Zech. 12:10, 11); in many of
   the psalms (51, etc.).

     Mourning was expressed, (1) by weeping (Gen. 35:8, marg.; Luke
   7:38, etc.); (2) by loud lamentation (Ruth 1:9; 1 Sam. 6:19; 2
   Sam. 3:31); (3) by the disfigurement of the person, as rending
   the clothes (Gen. 37:29, 34; Matt. 26:65), wearing sackcloth
   (Gen. 37:34; Ps. 35:13), sprinkling dust or ashes on the person
   (2 Sam. 13:19; Jer. 6:26; Job 2:12), shaving the head and
   plucking out the hair of the head or beard (Lev. 10:6; Job
   1:20), neglect of the person or the removal of ornaments (Ex.
   33:4; Deut. 21:12, 13; 2 Sam. 14:2; 19:24; Matt. 6:16, 17),
   fasting (2 Sam. 1:12), covering the upper lip (Lev. 13:45; Micah
   3:7), cutting the flesh (Jer. 16:6, 7), and sitting in silence
   (Judg. 20:26; 2 Sam. 12:16; 13:31; Job 1:20).

     In the later times we find a class of mourners who could be
   hired to give by their loud lamentation the external tokens of
   sorrow (2 Chr. 35:25; Jer. 9:17; Matt. 9:23).

     The period of mourning for the dead varied. For Jacob it was
   seventy days (Gen. 50:3); for Aaron (Num. 20:29) and Moses
   (Deut. 34:8) thirty days; and for Saul only seven days (1 Sam.
   31:13). In 2 Sam. 3:31-35, we have a description of the great
   mourning for the death of Abner.