The phrase "us against them" originates from the fundamental human cognitive tendency to form in-groups and out-groups, a concept deeply studied in social psychology. It lexicalized into a hyphenated compound adjective in the 20th century, reflecting its use in political, sociological, and journalistic discourse to succinctly describe divisive tactics or polarized mindsets. The logic is entirely transparent and literal, drawing directly from the core meanings of the English pronouns "us" and "them" and the preposition "against," to encapsulate a worldview defined by conflict between a collective self and a perceived other. Its power lies not in classical roots but in the direct, stark opposition of these basic English terms.

This is a fixed idiomatic phrase used attributively as a compound adjective. It is not productively formed from separable, recombinable morphemes in the modern sense but is a lexicalized combination of common English words.