While from anyone else, that self-description might seem like a person or character acting politely humble, Portia seems to be genuinely humble. She has the confidence that accompanies great beauty and wealth, but she is still nervous about marrying the right man, and is not too proud to fall in love with and marry Bassanio, despite his lack of fortune. She is also generous with her fortune--when Bassanio admits that his friend is in debt for the considerable sum of 3,000 ducats, she immediately offers enormous amounts of money to help him rescue Antonio from Shylock: “What, no more? Pay him six thousand, and deface the bond; double six thousand and then treble that.” (III.ii.306-9)
Portia is definitely feisty and intelligent (and humble, to boot!), but she could well be all of those other things--'an unlessoned girl, unschooled, unpractised'--as well.
One of the great mysteries of The Merchant of Veniceis how much help Portia gets from her cousin Bellario, a famous legal expert from Padua. The audience is never entirely sure whether Bellario used his years of schooling and legal expertise to help Portia come up with the courtroom strategies she uses as Balthazar or if Portia had insights all her own. Directors could choose to stage it one way or another, but the text leaves this ambiguous. If Bellario came up with the idea for how to beat Shylock and relayed that to Portia, then Portia could well be 'unlessoned' and 'unschooled', raised, as many wealthy Renaissance women were, to manage a household and children rather than given formal schooling.
In The Girlhoods of Shakespeare's Heroines, a series of novellas written about Shakespeare's heroines as girls written by Victorian Shakespeare critic Mary Cowden Clarke, the author imagines that Portia would have been discouraged from pursuing formal legal study by her uncle Bellario. Cowden Clarke has Bellario say to the young Portia that "women, though shrewd and quick judging, are apt to jump too rapidly at conclusions, and mar the power of their understanding by its too vivacious action." Her young Portia disagrees absolutely, confidently, and politely:
"One day or another you may be brought to acknowledge that I could make a profound lawyer," replied the smiling Portia; "am I not your disciple? and must not the pupil of the learned Doctor Bellario needs become so if she chooses?"*
Bellario replies that she should only study as much law as is necessary to run her estate well. But readers of Cowden Clarke's charming and thoughtful novellas would likely be familiar with Shakespeare's play, and would know that Portia ultimately does indeed make a "profound lawyer." In Cowden Clarke's expert interpretation, at least, Portia doesn't--and can't--count on Bellario's assistance for her legal education beyond the absolute basics.
Still, no matter who pored over the books of law and developed her court tactics, Portia displays courage and intelligence in presenting herself as Balthazar in order to rescue her new husband's closest friend.
*Mary Cowden Clarke, Portia, the Heiress of Belmont (New York, G.P. Putnam, 1878) p. 52.
In Professor Barnet's Signet edition of ROMEO AND JULIET we find notes from Samuel Johnson: "Here is one of the few attempts of Shakespeare to exhibit the conversation of gentlemen, to represent the airy sprightliness of juvenile elegance." Certainly there were more "attempts." What you quote in the question consists of three terms where someone like myself might say "I don't know jack." We have seen here that Portia's speech is an example of modesty, such as we find from Juliet: "Think true love acted simple modesty"(ROM, Act 3, scene 2) and "Not stepping o'er the bounds of modesty"(4.2). Also, in this play more than others, many have noted various thoughts are implied in one speech and not further explained. So, here Portia may well have more learning than we might guess at. One implication, then, is that she is looking forward to her future with Bassanio and getting to know him. In the first scene of the play, Bassanio says only that "Sometimes from her eyes / I did receive fair speechless messages"(1.1.166 or so). In the court scene, the Duke says: "The other half comes to the general state, / Which humbleness may drive unto a fine"(4.1.387 or so). Editors tend to gloss "humbleness" to mean "remorse(on Shylock's part)"(RSC edition). One might argue that the Duke is rather implying that a gentleman must, like Portia, sometimes be modest or humble.