In Act3 Scene2, from the extract, " and yet a maiden hath no tongue, but thought, - ............ The other half yours, -
What is meant by the first line, " and yet a maiden hath no tongue, but thought"?
Shakespeare's Portia is extremely theatrical, and this passage certainly exemplifies her penchant for histrionics. (Interestingly, there is a Histrionic Personality Disorder which identifies many of Portia's characteristics, one of which is the desire for much attention.)
In this passage, Portia expresses frustration that she "hath no tongue," no say in her future, but she can use words [her thoughts] to, hopefully, delay Bassanio, recognizing that he is no Romeo who will fall instantly in love. Because she cannot reveal which casket is the one that will win her--"she hath no tongue" in this matter--and because she can exert no force that will cause an inexperienced lover like Bassanio to fall in love with her, she seeks to delay him that he might become more interested as he becomes more knowledgeable of her in the employment of her "thought." That is, when Portia says, "and yet a maiden hath no tongue, but thought," she means that she understands that she cannot reveal the secret of the caskets to win Bassanio, but she has a mind which also understands that he is a worldly man, one who enjoys pleasure, but also one who is scholarly. So, with her "thought," she hopes to interest Bassanio and intrigue him with her noble mind, which may, hopefully, cause him to love her.
The passage of Portia cited that is under discussion is footnoted in A New Valorium Edition of Shakespeare: The Merchant of Venice. 10th edition. 1888, footnote #2:
....Nevertheless with the high instinct of her noble heart and intelligence, she doubts not that a man of his endowments will not fail to love her truly if he once but know her thoroughly.
(Hope this helps you)