Literally 'to lose one's grasp on' — from OE for- (away) + getaną (to seize), memory conceived as a grip that slips.
To fail to remember; to be unable to recall or think of something.
From Old English 'forgietan' meaning 'to forget, to lose from memory, to neglect,' from Proto-Germanic *fer-getaną, a compound of *fer- (away, completely, destructively) and *getaną (to grasp, to seize, to get). The original image is strikingly physical: to forget is to 'lose one's grasp on' something — what you once held in the mind slips away. The prefix 'for-' here has a destructive or reversing sense (as in 'forbid,' 'forsake,' 'forlorn'), so forgetting is the un-grasping of what was once grasped. Key roots: *fer- (Proto-Germanic: "away, completely, destructively"), *gʰed- (Proto-Indo-European: "to seize, to grasp, to take").
The word 'forget' is literally 'for-get' — 'to lose one's GET on something,' where 'get' originally meant 'to grasp.' German makes the same construction transparent: 'vergessen' is 'ver-' (away) plus the root of 'essen' is unrelated, but the 'ver-gessen' follows the exact same 'away + grasp' pattern from the same Proto-Germanic root. The flower 'forget-me-not' gets its name from a medieval German legend in which a drowning knight threw the flower to his beloved, crying 'Vergiss mein nicht!' (Forget me not!).