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.
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            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.
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            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