From Latin 'adversus' (turned against) — something adverse is literally facing you in opposition.
Preventing success or development; harmful; unfavourable; acting against or in opposition.
From Old French avers (hostile, contrary), from Latin adversus (turned against, opposite), the past participle of advertere (to turn toward), composed of ad- (toward) + vertere (to turn). Latin vertere derives from PIE *wert- (to turn, wind), one of the most productive Indo-European roots, yielding Latin versus (turned, a line of writing — each line being a "turn" of the plow), German werden (to become — "to turn into"), English -ward (as in "toward," "backward"), and Russian vertet' (to spin). The semantic development of adversus is spatial: something "turned toward" you in opposition, facing you as an enemy faces you in battle. In Latin, adversus served
'Adverse' and 'averse' are often confused but are etymologically distinct despite sharing the same root. 'Adverse' (from 'adversus,' turned against) describes external conditions that oppose you — adverse weather, adverse effects. 'Averse' (from 'aversus,' turned away) describes an internal attitude of reluctance — you are averse to something when you turn away from it. One is turned against you; the other is you turning away.