Use 'not half' when you want to show you're surprised by how much or to what extent something is true.

Expressing Surprise

'Not half' can hint that you're impressed or approve of something, often in an understated way.

Understated Approval

'Not half' can amplify a quality to show intensity, similar to 'very' or 'extremely.'

Use 'not half' to make a comparison that highlights a significant difference in quality or quantity.

'Not half' can convey a tone of disappointment or dissatisfaction when something isn't as expected.

When you're underwhelmed or unimpressed by something, use 'not half' to express this feeling.