"The love that dare not speak its name" is a euphemism for homosexual love.
It has its origins in Two Loves, an 1894 poem by Lord Alfred Douglas:
Tell me why, sad and sighing, thou dost rove
These pleasent realms? I pray thee speak me sooth
What is thy name?' He said, 'My name is Love.'
Then straight the first did turn himself to me
And cried, 'He lieth, for his name is Shame,
But I am Love, and I was wont to be
Alone in this fair garden, till he came
Unasked by night; I am true Love, I fill
The hearts of boy and girl with mutual flame.'
Then sighing, said the other, 'Have thy will,
I am the love that dare not speak its name.'
The term is most famous from Oscar Wilde's trial, in 1895. When the prosecutor asked him what he meant by 'the love that dare not speak its name', Wilde responded:
"The Love that dare not speak its name" in this century is such a great affection of an elder for a younger man as there was between David and Jonathan, such as Plato made the very basis of his philosophy, and such as you find in the sonnets of Michelangelo and Shakespeare. It is that deep, spiritual affection that is as pure as it is perfect. It dictates and pervades great works of art like those of Shakespeare and Michelangelo, and those two letters of mine, such as they are. It is in this century misunderstood, so much misunderstood that it may be described as the "Love that dare not speak its name," and on account of it I am placed where I am now. It is beautiful, it is fine, it is the noblest form of affection. There is nothing unnatural about it. It is intellectual, and it repeatedly exists between an elder and a younger man, when the elder man has intellect, and the younger man has all the joy, hope and glamour of life before him. That it should be so, the world does not understand. The world mocks at it and sometimes puts one in the pillory for it.
This speech caused an outburst of applause in the courtroom.