A person you think is strange — in how they dress, how they act, or what they say. It is informal and it is an insult, though a mild one. Between friends who are joking, 'you weirdo' can be almost affectionate, the way you might laugh at a friend who eats ice cream with chopsticks. Said about a stranger, it is not affectionate at all: 'some weirdo was following me' means you felt uncomfortable, maybe a little frightened. Because of this second use, be careful — calling someone a weirdo to their face, when you are not close, is rude and can hurt. There is a real cruelty in the word when it is aimed at someone who is different because of who they are rather than what they chose. It is a countable noun: a weirdo, two weirdos. Plural is spelled with -s (weirdos), not -es. Some people also use it as an adjective, as in 'weirdo music', but that is much less common and sounds quite casual. The word is fairly young — it appeared in American English around the 1950s — so it still sounds modern and spoken, not literary.