口 kǒu， a square-shape Chinese character, means mouth. It seems that Chinese people find more meaningful to represent the mouth with its opened shape. Very logical, we need to open the mouth to eat 吃 chī, to drink 喝 hē and to sing 唱 chàng. As you can see, 口kǒu is the radical of these three verbs in Chinese.
樱桃小口 yīng táo xiǎo kǒu, cherry-like small mouth, is considered as the traditional female beauty in China. Beautiful women in the paintings of Tang Dynasty are always fleshy, elegant and their mouths are cherry red and tiny.
人口 rénkǒu，man-mouth, means population. When we count the population, we are mainly counting the mouths to feed. 你家 nǐjiā 有yǒu 几 口 人 jǐ kǒu rén？Your family has how many people? Same logic of mouth-counting in the family. 吃 一口 chī yī kǒu，eat a bite, 喝 一口 hē yī kǒu, drink a sip, this is basic and natural way to mesure the F&B quantity inside of our mouth, wether it is huge or tiny.
门口 mén kǒu, door’s mouth=entry, 路口 lù kǒu, road’s mouth=intersection, both indicate a starting point. Chinese people consider the mouth as the starting point of some troubles: 病 从 口 入 bìng cóng kǒu rù, sickness via the mouth comes in，this idiom reminds us to pay attention to what we eat; 祸 从 口 出 huò cóng kǒu chū, trouble via the mouth comes out, this expression teaches us the misfortune caused by inappropriate words.
Nevertheless, for Chinese people, the mouth is at first where the happiness comes in. 口福 kǒu fú, the mouth-happiness, 有 口福 yǒu kǒu fú, to have the mouth-happiness, 大饱口福 dà báo kǒu fú, grandly fulfil the mouth-happiness, all these expressions indicate the wonderful opportunities to enjoy the delicious food. 祝你 zhù nǐ, wish you 有口福 yǒu kǒu fú！
Xiao Ling and Lao Long explain more details in the video hereunder.