sucks, slang to be objectionable or inadequate.
"It sucks" means "It is not good. "You suck" means that "you" are not good. Source
suck, Old English sucan, corresponding to Latine sugere "to suck." It's of imitative origin. Meaning "do fellatio" is first recorded 1928. Slang sense of "be contemptible" first attested 1971 (the underlying notion is of fellatio). Source
Note from Casiopea: For those who know the origin of "sucks" (i.e., that is stems from the act of fellatio, notably women who did the act), the use of slang "sucks" is objectionable because its meaning houses a history); however, for those who do not know its origin, "sucks" is not considered objectionable. :wink: Note also, using "sucks" to refer to a person can be taken two ways: 1) The teacher sucks (S/he is a boring/a bad teacher) or 2) The teacher sucks (S/he performs a contemptible act--look to its origin). :wink:
When Electroluux first marketed their vacuum cleaners in the U.S., their slogan was, "Nothing sucks like an Electrolux!" Apparently, the Swedish-speaking people who created that slogan didn't know that in American slang, "suck" also means "to be bad".
Rockin' Rudy writes in response:
"Sucks" isn't a swear word. If something "sucks", that means it isn't good. For example, "That TV show really sucks." You don't live in America, do you? Source
All the best, :D