當衰 is the original term that means "being unlucky". But 衰 [seoi1] is usually considered an inauspicious word. 垂 [seoi4] is a possible euphemism that sounds near to 衰, but it cannot be used in colloquial Cantonese. Then comes 髧 [dam3], it is colloquial and carries the same meaning of "hanging down” as 垂, but it is not common at all in usage. So 揼 [dam1], a near homophone of 髧, is generally chosen to replace 衰. See also [www.cantonese.sheik.co.uk]