By surface analysis, mourn +‎ -ing.

• (General American) IPA^(key): /ˈmoɹnɪŋ/, [ˈmo̞ɹnɪŋ]; (rare) IPA^(key): /ˈmʊɹnɪŋ/

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• (Received Pronunciation) IPA^(key): /ˈmɔːnɪŋ/; (rare) IPA^(key): /ˈmʊənɪŋ/
• (without the horse–hoarse merger, rhotic) IPA^(key): /ˈmo(ː)ɹnɪŋ/, (rare) /ˈmʊɹnɪŋ/
• (without the horse–hoarse merger, non-rhotic) IPA^(key): /ˈmoənɪŋ/, (rare) /ˈmʊənɪŋ/
• Rhymes: -ɔː(ɹ)nɪŋ
• Homophones: morning (horse–hoarse merger); moaning (dough–door merger)

mourning (countable and uncountable, plural mournings)

1. The act of expressing or feeling sorrow or regret; lamentation.
2. A feeling or expressing sorrow over someone's death.
   • 1900 May 17, L[yman] Frank Baum, chapter 23, in The Wonderful Wizard of Oz, Chicago, Ill.; New York, N.Y.: Geo[rge] M[elvin] Hill Co., →OCLC:

     "My greatest wish now," she added, "is to get back to Kansas, for Aunt Em will surely think something dreadful has happened to me, and that will make her put on mourning; and unless the crops are better this year than they were last, I am sure Uncle Henry cannot afford it."

   • 1941 June, “Notes and News: The Derelict Glyn Valley Tramway”, in Railway Magazine, page 279:

     A blind bearing the monogram G.V.T. is pulled down over the waiting room window as if still in mourning for the passing of the railway.

   • 1959, Georgette Heyer, chapter 1, in The Unknown Ajax:

     And no use for anyone to tell Charles that this was because the Family was in mourning for Mr Granville Darracott […]: Charles might only have been second footman at Darracott Place for a couple of months when that disaster occurred, but no one could gammon him into thinking that my lord cared a spangle for his heir.

   • 2011 December 19, Kerry Brown, “Kim Jong-il obituary”, in The Guardian:

     Unsurprisingly for a man who went into mourning for three years after the death in 1994 of his own father, the legendary leader Kim Il-sung, and who in the first 30 years of his political career made no public statements, even to his own people, Kim's career is riddled with claims, counter claims, speculation, and contradiction. There are few hard facts about his birth and early years.

3. The traditional clothes worn by those who mourn (in Western societies, typically coloured black).
   • 1992, Hilary Mantel, A Place of Greater Safety, Harper Perennial, published 2007, page 88:

     ‘I'm bored. I can't go out anywhere because it's too soon and I have to wear this disgusting mourning.’

4. A drape or covering associated with mourning.
   • 1700, [John] Dryden, “Palamon and Arcite: Or, The Knight’s Tale. In Three Books.”, in Fables Ancient and Modern;[…], London: […] Jacob Tonson,[…], →OCLC:

     The houses to their tops with black were spread, / And ev'n the pavements were with mourning hid.

mourning

1. present participle and gerund of mourn