Early decks often refer to this card as the traitor.
Eliphas Levi supposes that this is the prudence card. As I have said before, in the decks which include the prudence card, the Popess is distinctly absent she is also commonly depicted with a book, just like prudence, and is the appropriate gender.
The way Eliphas Levi describes the character it is as if his body would form a rare symbol for phosphorous beneath the tau cross. The symbol for phosphorous more often rendered with the apex of the triangle point upwards is often referred to as the golden dawn. A stage of alchemy where the alchemist’s work finally proves fruitful. The term phosphorous actually means light-bringing. So as much as it is a chemical it might be referring to an enlightening stage of spiritual alchemy.
Some including Waite have attempted to suppose a fylfot cross in his legs possibly the Hindu sun god, Savitr or Surya represented by the Swastika. (blasphemously coopted by murderers in WWII). This is congruent with the halo / nimbus around his head emitting light and associating him with sun gods.
Consistent with the many sun gods he is possibly being sacrificed for the greater good and symbolizes martyrdom. While I would expect Waite to suggest a connection between the hanged man and Jesus, instead he says that the Hanged Man is not suffering, but instead in a state of contemplation which is why he asked for the face to be so calm and surreal. This is inconsistent with older decks where the hanged man is called the traitor and is a martyr to maintain moral order.
For Thelemites, this card symbolized the initiatory power of the Old Aeon. The Aeon of the Father or of Osiris, where a person experiences enlightenment through suffering and self-sacrifice. This is different to the current Aeon where enlightenment is achieved through individuation like a child vested with all authority growing to be their true self.
Interpretation
Convicted of a crime against an unusual law, a person is condemned to be hung upside down. The world seems topsy turvy for him for a while and everything is at peace, until eventually suffering befalls him and he realises his fate. He must go through this suffering publicly to prevent others from experiencing the same fate. It is for the greater good that he must take on a difficult fate, but soon it will be over.