A stoner is someone who smokes marijuana often — not once at a party, but as a regular habit. The word is slang and informal, so it belongs in conversation between friends, never in an essay or a news report. It carries a picture with it: someone slow, relaxed, a bit forgetful, laughing at things that are not very funny, sitting on the sofa. Whether that picture is fair is another question, but that is the picture the word paints. The tone depends completely on who is speaking. Among young people it is often light, even affectionate — a friend calls another friend a stoner and nobody is angry. From a parent, a teacher or an employer it is an insult, and it means 'this person is wasting their life'. There is also an adjective use you will meet in film reviews: a 'stoner comedy' or 'stoner movie' is a silly, funny film made for people in that mood. Because attitudes to marijuana are changing in some countries, the word is softening, but it is still slang and still a little rude about the person. Do not confuse this with the older, unrelated meaning where -er is just a measurement: 'a ten-stoner' meant a person weighing ten stone (a British weight). Nobody says that now.