"The movement in her hips strikes the hour when the poison sets in.."
Direct reference to the sexual act going on between her and his friend. The movement in her hips tells of when the "poison," most likely referring to an alcohol aided lapse in her own judgement. Thus labeling it a "poison" as what it is facilitating as being toxic and painful to him.
Girlfriend cheating on him with best friend, and if not cheating then leaving him for his best friend.
"The movement in her hips strikes the hour when the poison sets in.."
Girlfriend cheating on him with best friend, and if not cheating then leaving him for his best friend.