Semantic shifting goes by many different names. In his pioneering article, Benedict 1939 calls it "semantic differentiation." In his article on Gnau,… - James Alan Matisoff

Bilingual humor may be viewed as a relatively benign response to the frustrations inherent in living in such a linguistically and racially heterogeneous region as the hills and valleys of backwoods Southeast Asia.

In the Beginning was the Sino-Tibetan monosyllable, arrayed in its full consonantal and vocalic splendor. And the syllable was without tone and devoid of pitch. And monotony was on the face of the mora. And the Spirit of Change hovered over the segments flanking the syllabic nucleus.
And Change said, "Let the consonants guarding the vowel to the left and the right contribute some of their phonetic features to the vowel in the name of selfless intersegmental love, even if the consonants thereby be themselves diminished and lose some of their own substance. For their decay or loss will be the sacrifice through which Tone will be brought into the world, that linguists in some future time may rejoice."
And it was so. And the Language saw that it was good, and gradually began to exploit tonal differences for distinguishing utterances – yea, even bending them to morphological ends. And the tones were fruitful and multiplied, and diffused from tongue to tongue in the Babel of Southeast Asia.

It is a remarkable fact that a tremendous spate of tonogenetic and registrogenic activity occurred all over the South-East Asian linguistic area in the twelfth and thirteenth centuries, triggered by the devoicing of the previously *voiced series of obstruents in many Middle Chinese and Hmong-Mien dialects, in Siamese and other Tai languages, in Karenic, in Burmese and many Loloish languages, and in Vietnamese, Khmer, and other Mon-Khmer languages. It is interesting to note that this period was roughly contemporaneous with the Mongol invasions that convulsed Eurasia in those centuries. Is it going too far to regard these extralinguistic events as a sort of punctuation in the sense of Dixon (1997), a period of upheaval that shook up a previously stable prosodic constellation in South-East Asia? Could the peoples of the region have been so terrified by the Golden Hordes that they hardly dared to vibrate their vocal cords, dooming the voiced obstruents to transphonologize into mere breathy voice or lower tone?