Antiphrasis is a figure of speech in which a word or phrase is used to mean the opposite of its normal meaning to create ironic humorous effect. From the Greek : anti "opposite" and phrasis, "diction". The adjective form is antiphrastic.

Examples:

He's only a child of 50 years old.She's so beautiful.

She has an attractive long nose.

It is a cool 45 degrees Celsius in the shade.