I have been reading some blogs over the Internet that describe the word mleccha (म्लेच्छ) e.g., here.
Can someone explain me what mleccha actually means in the context of Hinduism and Hindu scriptures?
Mleccha is a Sanskrit word which means 'barbarian, uncultured, foreigner, non-Vedic'.
Aloka Parashara in his book, Mlecchas in Early India describes it as,
Mleccha (and its equivalent milakkha) are usually translated as foreigner or barbarian. A translation which is inadequate in so many ways but not least because it implies that it was a word used by Indians to describe non-Indians. In fact it is a term used by some writers who lived in certain parts of India to describe people native to what we think of as India but who lacked some important criteria the writer felt defined his cultural identity (language, religion, geographical location, ancestry etc.)
He describes the etymology of the word as,
As the earliest reference occurs in the Satapatha Brahmana, which is part of an oral tradition dating to before 500 BC, scholars have usually looked for various origins in the bronze age societies of the first and second millennium BC.
He also says,
...that language should feature heavily in the definition of Mleccha. In fact in early texts it is clear that mleccha status was defined largely in terms of language (either the inability to use Sanskrit, or the inability to use it correctly). Language was central to identity in ancient India, as evidence by the process of Sanskritization in the early centuries AD, the importance of the Grammarians from Panini onwards.
Mlechhas were mainly very context dependent and might have included Bactrians, Greeks, Huns, Scythians, and Kushans; while leftists argue that non-Vedic Indians like tribals were also included in this term.
René Daumal (1908-1944), a French scholar who learned sanskrit when he was in his 20 years, wrote in his book, "Bharata. The origin of theater. Poetry and music in India" (my translation) : "The Sanskrit mletcchâs, 'foreigners, barbarians' literally means 'bafouilleurs'". This last word, which I do not know the English translation, means someone who mispronounce words
Bharata. L’origine du théâtre. La poésie et la musique en Inde.
Those who have deserted practising their own duties, who are unkind and troublesome to others, and who are very excitable, envious and foolish are Mlechchhas. - Sukra Niti (Sukracharya's system of morals) 1.87-88
(References are to lines, not to Slokas)
Duties mean the prescribed duties of four Varnas, I guess. Because in the previous verses, the four Varnas were mentioned.
tatraitāmapi vācamūduḥ | upajijñāsyāṃ sa mlecastasmānna brāhmaṇo mlecedasuryā haiṣā vā natevaiṣa dviṣatāṃ sapatnānāmādatte vācaṃ te'syāttavacasaḥ parābhavanti ya evametadveda
24. Such was the unintelligible speech which they then uttered,--and he (who speaks thus) is a Mleccha (barbarian). Hence let no Brahman speak barbarous language, since such is the speech of the Asuras. Thus alone he deprives his spiteful enemies of speech; and whosoever knows this, his enemies, being deprived of speech, are undone.
mlecchas are not to be confused with dasyus who are defined as people outside varnashrama (irrespective of language they speak) in manu smriti :
45. All those tribes in this world, which are excluded from (the community of) those born from the mouth, the arms, the thighs, and the feet (of Brahman), are called Dasyus, whether they speak the language of the Mlecchas (barbarians) or that of the Aryas.