What is the term for a feeling of animosity or ill-will?

The term that denotes a feeling of animosity or ill-will is "animus." This word reflects a hostile or unfriendly attitude toward someone or something, often stemming from feelings of resentment or opposition. In many contexts, "animus" captures the essence of an underlying animosity, making it a fitting choice for describing feelings that are negative or antagonistic.

Other terms listed, while they have their own specific meanings, do not convey the same nuance of animosity. For example, "assiduous" refers to diligence and carefulness, "bellicose" describes a tendency to be aggressive or quarrelsome, but does not necessarily capture the emotional aspect of ill-will, and "antipodal" relates to being diametrically opposite, which does not pertain to feelings of animosity at all. Therefore, "animus" remains the most accurate term in this context.