That same wicked bastard of Venus, that was begot of thought, conceived of spleen, and born of madness, that blind rascally boy that abuses everyone’s eyes because his own are out, let him judge how deep I am in love. (181-84)
In Roman mythology Venus, the goddess of love and beauty, was married to Vulcan, the god of fire and the blacksmith to the gods. However, Venus had an affair with Mercury, the god of eloquence, travelers, and luck. The child from that sexual relation was Cupid, the god of love. Since Cupid is an illegitimate child, Rosalind refers to him as a bastard; and since he is the offspring of an act of adultery, Rosalind calls him wicked. During the Renaissance the common belief was that the spleen (not the heart) was the organ that produced hot passion and violent emotions in people. Rosalind naturally plays on the common conception that love is a form of madness. Thus, Cupid was born from an act of madness, is mad himself, and causes others to become equally mad. Finally, Rosalind also employs the common image of Cupid as a blind god. Love is blind: people who are in love cannot see clearly or think rationally.
Thus, Rosalind is declaring her own mad state. She cannot think clearly or sanely because of her affection for Orlando.