Valmiki Ramayana Sundara Kanda Sarga 59 Shloka 4

ताम् her, स्पृशतः with mere touch, तस्य of her, गात्रम् body, तपसा by penance, न विनाशितम् not destroyed, क्रोधकलुषीकृता if enraged (she) can destroy, जनकस्य Janaka's, आत्मजा daughter, यत् such, कुर्यात् she can do, तत् that, पाणिना by hand, संस्पृष्टासती touched with hand, अग्निशिखा like flame of fire, न कुर्यात् not done so.

"Sita can burn Ravana with the mere touch of her body but because of the power of his penance, he was not burnt even though he touched her hand. If enraged, Janaki can burn the whole world (by virtue of her chastity) in a way that even the flame of fire can not do. (But she desired that her husband should kill him.)