means A sudden interruption of blood flow to part of the brain — by a clot or a bleed — that starves brain tissue and can rapidly cause weakness, slurred speech, or worse.

from From Old English 'strac,' a blow or stroke of the hand, tied to the verb 'strican,' to move lightly over or strike — a cousin of German 'streichen' and the same root that gives us 'strike.' For centuries it just meant a hit or a sudden movement, then by around the 16th century it took on the sense of a sudden affliction, as though the body had been struck down 'by the hand of God.' That older sense survives in 'a stroke of luck' — a blow from fate, for better or worse.