Translation guide
Describes a person, plan, or expectation that is unrealistically hopeful or positive, often ignoring potential problems.
To say that someone is too hopeful or positive, often in a naive or unrealistic way.
Directly means 'too optimistic'. A straightforward and common way to describe someone.
I think he's overly optimistic.
To say that a plan, estimate, or expectation is unrealistically hopeful and likely to fail.
Same as for people, but applied to plans or views.
甘い implies naivety or lack of rigor, often used for plans or judgments. 楽観的すぎる simply means 'too optimistic' and can be used for people or situations without the nuance of naivety.
His estimate is naive/overly optimistic.
Literally 'sweet', but commonly used to mean 'naive' or 'overly optimistic' about a situation or person.
His thinking is overly optimistic.
Literally 'auspicious', but used sarcastically to mean 'naively optimistic' or 'simple-minded'.
That guy is overly optimistic (naive).
Describes a carefree, overly optimistic person who doesn't worry enough. Often slightly negative.
He's way too carefree and optimistic.
An 'optimist' by nature. Can be neutral or slightly negative if implying unrealistic optimism.
He's an optimist, so he doesn't worry.
That plan is overly optimistic.
'Naive outlook' or 'overly optimistic forecast'. Common in business contexts.
That's an overly optimistic forecast.
'Wishful thinking' or 'optimistic speculation'. Often used when someone is basing hopes on desires rather than facts.
That's just wishful thinking.
The outlook is naive/overly optimistic. A set phrase.
His outlook is overly optimistic.
He is overly optimistic.
Do not directly translate 'overly optimistic' as 過度に楽観的 (kado ni rakkanteki). While understandable, it sounds unnatural. Use the phrases above instead.