bear in mind

late 12c., from Old English gemynd "memory, remembrance, state of being remembered; thought, purpose; conscious mind, intellect, intention," Proto-Germanic *ga-mundiz (cf. Gothic muns "thought," munan "to think;" Old Norse minni "mind;" German Minne (archaic) "love," originally "memory, loving memory"), from PIE root *men- "think, remember, have one's mind aroused," with derivatives referring to qualities of mind or states of thought (cf. Sanskrit matih "thought," munih "sage, seer;" Greek memona "I yearn," mania "madness," mantis "one who divines, prophet, seer;" Latin mens "mind, understanding, reason," memini "I remember," mentio "remembrance;" Lithuanian mintis "thought, idea," Old Church Slavonic mineti "to believe, think," Russian pamjat "memory").

Meaning "mental faculty" is mid-14c. "Memory," one of the oldest senses, now is almost obsolete except in old expressions such as bear in mind, call to mind. Mind's eye "remembrance" is early 15c. Phrase time out of mind is attested from early 15c. To pay no mind "disregard" is recorded from 1916, American English dialect. To have half a mind to "to have one's mind half made up to (do something)" is recorded from 1726. Mind-reading is from 1882.