What is the ruling on touching one's maternal aunt's body over her clothes, and is this act considered adultery, and what is its expiation?

This act is forbidden. The excuse that this is not awrah (private parts) is a repulsive excuse. The preponderant opinion is that it is permissible for a man to see from his mahram (unmarriageable kin) what is commonly exposed. As for what is not commonly exposed, such as the back, stomach, and chest, it is awrah. Those who stipulated for the permissibility of looking the absence of fitna (temptation) and the lack of shahwa (lust) hold that it is not permissible to look or touch if lust is present or feared. What is obligatory is sincere repentance, increasing good deeds, and staying away from the maternal aunt. This act is among the precursors to zina (fornication/adultery) that necessitate disciplinary punishment, and it may be metaphorically called zina, as in the hadith: "Indeed, Allah has decreed for the son of Adam his share of zina. The zina of the eyes is looking, the zina of the tongue is speaking, the soul wishes and desires, and the private part confirms or denies all of that." These matters lead to actual zina.