सशक्त और सुस्पष्ट  =  argument
Of an argument- carefully prepared and written so that it contains no mistakes or weaknesses and is easy for everyone to understand

जलरोधी  =  close
A watertight compartment/joint/seal.

ठोस  =  strong
Of an excuse, etc- impossible to prove false

अकाट्य
A watertight alibi