(adj.) to describe a place where the business is very quiet / slow, with hardly any customers; literally 'squatting flies to kill time'.
Definitions (粵典–words.hk)
動詞
冇乜生意，冇乜人客 to have no, or only a few, customers; to have absolutely no, or not much, business; to run a slack business; literally: to swat a fly
沙士期間，好多舖頭都拍晒烏蠅，冇生意。
沙士期间，好多舖头都拍晒乌蝇，冇生意。
saa1 si2 kei4 gaan1, hou2 do1 pou3 tau2 dou1 paak3 saai3 wu1 jing1, mou5 saang1 ji3. During the SARS crisis, many shops ran a slack business, without any business.
Definitions (Wiktionary)
verb, Cantonese, verb-object
to swat flies (with a flyswatter)
verb, Cantonese, figuratively, of business, usually, verb-object
to be slack (to the extent that clerks can only swat flies in the shop); to be sluggish; to be quiet