Disgustingly disagreeable. There's some suggestion the slang term originates from early Jazz tradition in the 1920s when musicians said players who had a good sound when playing horns could really blow; hence 'he doesn't blow, he sucks' to be bad and thence into common usage. Although that's possible apocryphal to allow it into dictionaries and the like without having to make references to fellatio, although I've never really understood why 'cocksucker' is considered an insult. Maybe because it's incorrectly seen as a submissive act and our culture rates power and dominance as features of a successful person.

...on occasion I talk too much, you should feel free to stop me at any time.