warn v.

Proto-Indo-European root meaning "to cover."

It forms all or part of: aperitif; apertive; aperture; barbican; cover; covert; curfew; discover; garage; garment; garnish; garret; garrison; guarantee; guaranty; kerchief; landwehr; operculum; overt; overture; pert; warn; warrant; warrantee; warranty; warren; wat; Wehrmacht; weir.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vatah "enclosure," vrnoti "covers, wraps, shuts;" Lithuanian užveriu, užverti "to shut, to close;" Old Persian *pari-varaka "protective;" Latin (op)erire "to cover," (ap)erire "open, uncover" (with ap- "off, away"); Old Church Slavonic vora "sealed, closed," vreti "shut;" Old Irish feronn "field," properly "enclosed land;" Old English wer "dam, fence, enclosure," German Wehr "defense, protection," Gothic warjan "to defend, protect."

"notice beforehand of the consequences that will probably follow continuance in some particular course" [Century Dictionary], Old English warnung, verbal noun from warnian (see warn (v.)).