It stands for Di-ma-sa meaning sons of big waters referring to Brahmaputra river (known as Dilao in Dimasa).
ɪt stˈændz fɔːɹ dˈaɪmˈɑːsˈɑː mˈiːnɪŋ sˈʌnz ʌv bˈɪɡ wˈɔːɾɚz ɹᵻfˈɜːɹɪŋ tə bɹˈɑːmɐpˌʌtɹɚ ɹˈɪvɚ nˈoʊn æz dˈɪlaʊ ɪn dɪmˈɑːsə.