The interjection 咳
An interjection (感叹词) is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction. It expresses feelings and emotions but does not name them. It is usually placed at the beginning of a sentence.
咳 
means:
1) sorrow, regret “oh”, “damn”.
means:
1) sorrow, regret “oh”, “damn”.Examples
咳！我昨天没去医院
Oh, I didn't go to the hospital yesterday!
hāi! wǒ zuótiān méi qù yīyuàn
咳，没想到！
Damn, I didn’t think about it!
hāi, méi xiǎngdào
2) a “Hey!” shout.
Examples
咳，请别跑了！
Hey! Please don’t run away!
hāi, qǐng bié pǎo le
咳，急着去哪里？
Hey! Where are you going in such a hurry?
hāi, jízhe qù nǎlǐ
咳 also can be used in songs for making rhythms such as “hey”, “yeah”. It can be pronounced as hāi, hài, hái.
Examples
我们工人有力量，咳！我们工人有力量！
Us working people are strong, oh yeah! Us working people are strong!
wǒmen gōngrén yǒu lìliàng, hāi! wǒmen gōngrén yǒu lìliàng