Crazy
A common, informal alternative, similar to 'nuts' or 'bonkers'.
A stronger, more formal word, like 'demented' or 'insane'. It works for both masculine and feminine.
The noun form, meaning 'craziness' or 'madness'. Used to describe a situation.
My brother is a little crazy.
What a crazy idea! I love it.
The traffic in the city is craziness.
They are nuts, they are always making jokes.
'Loco/loca' is universally understood and is the most standard term.
'Chiflado/a' is very common and informal. You might also hear 'safado/a'.
Besides 'loco', 'chapita' is common slang (e.g., 'estás medio chapita').
'Rayado/a' is a very popular slang term that can mean 'crazy', 'obsessed', or 'stressed out'.
'Loco/loca' and 'chiflado/a' are the most frequent terms.
'Pirado/a' and 'chalado/a' are very common informal terms, similar to 'nuts' or 'bonkers'.
Saying 'Mi amiga es loco.' — The correct form is 'Mi amiga es loca.' In Spanish, adjectives must match the gender of the noun they describe. 'Amiga' is feminine, so you must use the feminine adjective 'loca'.
Confusing 'ser' and 'estar'. — 'Ser loco' implies a permanent state of insanity. 'Estar loco' refers to a temporary state or acting crazy. For most situations, like 'You're crazy!', you should use 'estar': '¡Estás loco!'.
Translating 'crazy about' literally. — To say 'I'm crazy about you,' don't say 'Estoy loco sobre ti.' The correct preposition is 'por'. The right way is 'Estoy loco por ti'.