Completely mistaken

He thought he knew the subject, but his speech was out of left field: it was totally wrong

Pensava di conoscere l'argomento, ma nel suo intervento è andato completamente fuori strada. Era totalmente sbagliato

Be careful of what you're saying! You've no clue, no idea of what happened and you're way out in left field on that one!

Stai attento a quello che dici! Non hai nessuna idea di quanto è successo e sei completamente fuori strada!

This idiom certainly comes from the world of baseball, but the exact allusion to left field is uncertain. According to one theory, in many early ball-parks left field was larger than right field, so it was easier to lose balls and generate confusion in left field