From Old English (before 12th century), from Proto-Indo-European '*pleu-' ("to flow, to run"), from PIE *pleu- ("to flow, run").
To run away from a place or situation of danger; to escape.
From Old English 'flēon' (to flee, escape, avoid), from Proto-Germanic '*þleuhaną' (to flee, to run away), from PIE root *pleu- (to flow, to run). The deep root *pleu- meant 'to flow' — fleeing was conceived as flowing away, like water escaping. The same PIE root produced 'fly,' 'flow,' 'float,' 'flood,' and 'fleet' — all words involving rapid movement
'Flee,' 'fly,' 'flow,' 'float,' and 'flood' all trace back to the same PIE root *pleu- (to flow). The ancient Indo-Europeans saw no hard distinction between running away, flying through air, and flowing like water — they were all just 'moving fluidly.' This is why English still confuses the past tenses