Origin and history of weak-minded

"that which feels, wills, and thinks; the intellect," late 12c., mynd, from Old English gemynd "memory, remembrance; state of being remembered; thought, purpose; conscious mind, intellect, intention," Proto-Germanic *ga-mundiz (source also of Gothic muns "thought," munan "to think;" Old Norse minni "mind;" German Minne (archaic) "love," originally "memory, loving memory"), from suffixed form of PIE root *men- (1) "to think," with derivatives referring to qualities of mind or states of thought.

The meaning "mental faculty, the thinking process" is from c. 1300. The sense of "intention, purpose" is from c. 1300. From late 14c. as "frame of mind. mental disposition," also "way of thinking, opinion."

"Memory," one of the oldest senses, now is almost obsolete except in old expressions such as bear in mind (late 14c.), call to mind (early 15c.), keep in mind (late 15c.). The expression time out of mind "indefinite long period of time" is roughly from mid-14c. (tyme of whilk no mynd es), later, in English law, "before Richard I" (1189).

Mind's eye "mental view or vision, remembrance" is from early 15c. To pay no mind "disregard" is recorded by 1910, American English dialect. To make up (one's) mind "determine, come to a definite conclusion" is by 1784. To have a mind "be inclined or disposed" (to do something) is by 1540s; to have half a mind to "to have one's mind half made up to (do something)" is recorded from 1726. Out of (one's) mind "mad, insane" is from late 14c.; out of mind "forgotten" is from c. 1300; phrase time out of mind "time beyond people's memory" is attested from early 15c. 

My head is hands and feet. I feel all my best faculties concentrated in it. My instinct tells me that my head is an organ for burrowing, as some creatures use their snout and fore-paws, and with it I would mine and burrow my way through these hills. I think that the richest vein is somewhere hereabouts; so by the divining rod and thin rising vapors I judge; and here I will begin to mine. [Thoreau, "Walden"]

c. 1300, weik, "deficient in bodily strength; exhausted from exertion," also "lacking moral strength;" from Old Norse veikr "lacking strength," which is cognate with Old English wac "weak, pliant, soft."

They are reconstructed to be from Proto-Germanic *waika- "yield" (source also of Old Saxon wek, Swedish vek, Middle Dutch weec, Dutch week "weak, soft, tender," Old High German weih "yielding, soft," German weich "soft"), which according to Watkins is from PIE root *weik- (2) "to bend, to wind."

Of tools, etc., "lacking effectiveness," early 14c.; of things, "fragile, breakable," late 14c. Specifically of substances, medicines, "not sufficiently imbued with the usual qualities or ingredients," 1590s.

Of a person, especially an opponent or enemy, "lacking fighting skill, deficient in combat power," early 14c. The sense of "lacking authority over others" is recorded by late 14c.

In grammar, denoting a verb inflected by regular syllabic addition rather than by change of the radical vowel, from 1833, in contrast to strong (adj.).

Of the voice by early 14c. In reference to a pulse, "faint," by 1700. Related: Weakly; weaker; weakest.

Figurative expressions about a chain being no stronger than its weakest link are attested by 1846. Weaker vessel as a figurative phrase for "woman" is by 1520s; in Tyndale's New Testament weak translates Greek asthenōn, which St. Paul used in reference to any believer whose faith was beset by doubts and who should be treated tenderly and patiently. Compare weakling.