From Latin 'assistere' (to stand by) — originally meant simply 'to be present' (as it still does in French), but English shifted it to active help.
To help or aid someone in doing something; to provide support or relief.
From Old French "assister" (to stand by, to be present, to help), from Latin "assistere" meaning "to stand at, to stand near, to attend," composed of "ad-" (to, toward) and "sistere" (to cause to stand, to place). Latin "sistere" is a reduplicated causative form of "stāre" (to stand), which derives from Proto-Indo-European *steh₂- (to stand), one of the most productive roots in the language family. PIE *steh₂- generated an enormous reflexive network: Latin "stāre" (to stand), Greek "histēmi" (I cause to stand), Sanskrit "tiṣṭhati" (he