From way back, not baring one’s teeth in front of others when laughing and not opening one’s mouth wide and showing the inside of the mouth was one of the decorums of etiquette strictly observed by women.
It was strictly observed because from ancient times, women in Japan dyed their teeth black (ohaguro).
The custom of ohaguro began during the Nara period (710– 794) onward and it became popular among the aristocratic women in the Heian period (794– 1185). In the latter Heian, it was also practiced by men court nobles and the samurai. During the Muromachi period (1333– 1568), girls who reached nine years of age dyed their teeth as a mark of having attained adulthood. During the Edo period (1600– 1868), all married women dyed their teeth.
The black color was derived from soaking a piece of iron that was left to oxidize in a liquid of tea and vinegar. Hesitant about showing their blackened teeth, women hid their mouth with their hand or kimono sleeve. This was considered to be an act of courtesy that reflected refinement.
Non–Japanese seem to feel that covering the mouth when laughing is a sign of immaturity on the part of Japanese women. And indeed, the psychiatrist Freud has said that covering the mouth while laughing is an exhibition of infantile behavior.
Excluding elderly women, there are very few young women of today who cover their mouth when laughing.