adjectival noun / noun:

• unreasonable; unnatural; unjustifiable
• impossible

adjectival noun / noun / ~する noun:

• forcible; forced; compulsory - often as 無理に

adjectival noun:

• excessive (work, etc.); immoderate

~する noun:

• to work too hard; to try too hard

interjection:

• no way; not a chance; never; dream on - colloquialism

~の noun:

• irrational - Mathematics term