अन्वयः

कत्थनेन by boasting, तप्तेन heated, सुवर्णप्रतिरूपेण looking like gold, कुशाग्निना इव by the fire of kusa grass, सर्वथैव by all means, ते your, लघुत्वम् meanness, विदर्शितम् is exhibited.

M N Dutt

As brass wearing the semblance of gold, displays its own defect on being heated*, with a fire lit with Kusa, so you have betrayed your own lightness by this speech of yours. i.c. becomes dusky, (Ramanuja).

Summary

All your meanness is exhibited by your boasting just as the kusa grass caught by fire appears like gold.

पदच्छेदः

सर्वथा	सर्वथा (अव्ययः)
तु	तु (अव्ययः)
लघुत्वं	लघु–त्व (१.१)
ते	त्वद् (६.१)
कत्थनेन	कत्थन (३.१)
विदर्शितम्	विदर्शित (√वि-दर्शय् + क्त, १.१)
सुवर्णप्रतिरूपेण	सुवर्ण–प्रतिरूप (३.१)
तप्तेनेव	तप्त (√तप् + क्त, ३.१)–इव (अव्ययः)
कुशाग्निना	कुश–अग्नि (३.१)

छन्दः

अनुष्टुप् [८]

छन्दोविश्लेषणम्

१	२	३	४	५	६	७	८
स	र्व	था	तु	ल	घु	त्वं	ते
क	त्थ	ने	न	वि	द	र्शि	तम्
सु	व	र्ण	प्र	ति	रू	पे	ण
त	प्ते	ने	व	कु	शा	ग्नि	ना