The adjective “vexed” means to feel frustration, distress, or irritation. Yeats creates a subtle contrast between the turmoil described in the preceding stanza and the “stony sleep” that was “vexed to nightmare” over the last twenty centuries—since Christ ascended into heaven, according to Christian tradition. Thus, it seems that bloodiness and anarchy are symptomatic of the Second Coming as opposed to being its cause.
The adjective “indignant” means to experience or express anger or displeasure at what is perceived as unfair or unjust. Its use here suggests not only that the beast has disturbed the desert birds as it moves towards Bethlehem, but also that its presence is generally unjust, which sharply contrasts with the Christian idea of the righteousness and inevitability of the Second Coming.
The phrase “Spiritus Mundi” is Latin for “world spirit.” Yeats is engaging with the Platonic idea that all people are united in one collective mind and that a person’s imagination reflects the contents of this broad consciousness. Referring to Spiritus Mundi here thus lends significance to the “vast image” the speaker sees, for this vision is derived from humanity’s collective mind as opposed to one person’s subjective musings.
The noun “anarchy” means a state of social disarray, usually in the absence of—or the refusal to acknowledge—authority. Yeats’s understatement in the phrase “mere anarchy”—where “mere” takes on an archaic meaning of total or absolute—illustrates the dismal state of society while also suggesting that people are suffering because they lack an orderly authority figure such as God.
The noun “gyre” refers to a spiraling form. In this poem, Yeats portrays time as a turbulent force spiraling outward, carrying society forward in confusion and disorder.
