Dear tsuwm: for once I am not impressed by your references'
I remember hearing "cocksucker" shortened to "sucker" to "he sucks" over seventy years ago. There was also "sucker" meaning a gullible person. I don't believe "sucks" can be totally divorced from "cocksucker".
I don't doubt that a lot of people that have used it were not aware of that possibility. But that is similar to the number of times I have heard "scumbag" used by people who had copied it from someone else without knowing what it really meant.Don't try calling a cop that.

It may not be hazardous now, but I can remember when calling a Canadian a "bum" could get you a punch in the mouth.