In Act I, Scene iii of Othello, what is the warning that Brabantio gives to Othello?
In Othello, Iago's personal agenda is to destroy Othello and anyone and anything associated with him. Iago is one of the most enigmatic (as in incomprehensible and impenetrable), but nonetheless cunning and calculating characters, ever created. His ability to charm and apparently almost hypnotize everyone with whom he has any contact is what creates suspense and disbelief as he fools one character after the next. Even when there is doubt, he mitigates the circumstances until even a ridiculous notion becomes believable.
Iago persuades Roderigo that he will help him win over Desdemona despite the fact that she is married to Othello and that Brabantio, Desdemona's father, has previously rejected Roderigo's advances on his daughter as being without foundation, reminding Roderigo that she is "not for thee" (I.i.99).
Brabantio learns of his daughter's apparent betrayal from Roderigo's initial shouts, courtesy of Iago, and Iago quickly steps in to take control and convince him that Othello, whom Brabantio has welcomed into his home on many occasions, is nothing more than a "Barbary horse" (I.i.111). It takes quite some persuasion to convince him that Othello did not bewitch his daughter. Accordingly, even though Iago has been proven wrong, it is enough to create suspicion in Brabantio that he even doubts his own daughter's allegiances. He overlooks the fact that his own wife did a similar thing when she married him and warns Othello that "she has deceived her father and may thee" (I.iii.293). It is significant that the very next line mentions "honest Iago" (294).
This comment in line 294 shows that Iago is taken into Othello's confidence very early on and truly believes in the sincerity of "honest" Iago even telling Michael Cassio that he is "most honest" (II.iii.7). This belief together with Iago's "ocular proof" (III.iii.364) will drive the story forward to its tragic end.
After a dramatic confrontation with Othello and Desdemona, Brabantio bitterly accepts the truth that his daughter has chosen freely to be Othello's wife. He feels angry and betrayed, but he is powerless to impose his will upon her. In frustration, he says to Othello these words about Desdemona:
Look to her, Moor, if thou has eyes to see.
She has deceived her father, and may thee.
He is warning Othello that since Desdemona has deceived her own father, she may very well betray her husband. There is no truth in Brabantio's cruel, bitter comment; his daughter will be a loving and faithful wife. Brabantio's words, however, will play an important role as the drama develops, since Iago is standing nearby when Brabantio warns Othello about Desdemona's possibly deceiving and betraying him in the future.