2 Answers

In what context?

That’s so radical! 

radical |?radik?l|
adjective
1 (esp. of change or action) relating to or affecting the fundamental nature of something; far-reaching or thorough : a radical overhaul of the existing regulatory framework.
• forming an inherent or fundamental part of the nature of someone or something : the assumption of radical differences between the mental attributes of literate and nonliterate peoples.
• (of surgery or medical treatment) thorough and intended to be completely curative.
• characterized by departure from tradition; innovative or progressive : a radical approach to electoral reform.
2 advocating thorough or complete political or social reform; representing or supporting an extreme section of a political party : a radical American activist.
• (of a measure or policy) following or based on such principles.
3 of or relating to the root of something, in particular
• Mathematics of the root of a number or quantity.
• denoting or relating to the roots of a word.
• denoting the semantic or functional class of a Chinese character.
• Music belonging to the root of a chord.
• Botany of, or springing direct from, the root or stem base of a plant.
4 [usu. as exclam. ] informal very good; excellent : Okay, then. Seven o'clock. Radical!
noun
1 a person who advocates thorough or complete political or social reform; a member of a political party or part of a party pursuing such aims.
2 Chemistry a group of atoms behaving as a unit in a number of compounds. See also free radical . [ORIGIN: early 19th cent.: from French.]
3 the root or base form of a word.
• any of the basic set of 214 Chinese characters constituting semantically or functionally significant elements in the composition of other characters and used as a means of classifying characters in dictionaries.
4 Mathematics a quantity forming or expressed as the root of another.
• a radical sign.
DERIVATIVES
radicalism |-?liz?m| noun (in sense 1 of the noun) .
radically |-ik(?)l?| adverb [as submodifier ] : a radically different approach.
radicalness noun
ORIGIN late Middle English (in the senses [forming the root] and [inherent] ): from late Latin radicalis, from Latin radix, radic- ‘root.’