literall: missing one hair, i.e. to be harmed or hurt in any way at all [colloquial]
Definitions (粵典–words.hk)
語句
受到任何傷害 harmed or hurt in any way at all; literally: with one hair fewer
如果佢少咗一條毛，我唔會放過你。
如果佢少咗一条毛，我唔会放过你。
jyu4 gwo2 keoi5 siu2 zo2 jat1 tiu4 mou4, ngo5 m4 wui5 fong3 gwo3 nei5. If he gets harmed in any way at all, I will not let you go.