Why does it mention an adulteress?
Rashi: This is the evil that caused [the kingdoms to rule over Yisrael]. Yisrael were Mezanah with idolatry. This punishment is proper to come!
Malbim (Derech ha'Pashut): This refers to Semamis, a Shifchah of the evil king. He was Mezaneh with her, and concealed it; she betrayed him (she counseled her husband Alkum to kill the king and take his throne; she ruled in place of her mistress. Adultery is the fourth matter; it pertains to the fourth element (fire). It says "Lo Yedatim" (18), for also it leaves no mark.
What is the significance of eating?
Rashi: This is a euphemism [for Zenus]. Malbim (Derech ha'Muskal) - she 'eats', wipes her [lower] mouth and nothing remains [from the Zenus], and she can deny and say 'I did not sin'. Malbim (Derech ha'Pashut) she acts as if there was no fire of love [between her and the adulterer].
What is the meaning of "u'Machasah"?
Rashi: She bends her mouth down. This is like "Ka'asher Yimcheh Es ha'Tzalachas Machah v'Hafach Al Paneha" (Melachim II, 21:3).
What is the significance of saying 'I did not sin'?
Rashi: This is like "Hineni Nishpat Osach Al Amrech Lo Chatasi" (Yirmeyah 2:35).