mind-boggling adj.

"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.

Meaning "mental faculty, the thinking process" is from c. 1300. 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.).

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. 

1590s, "to start with fright (as a startled horse does), shy, take alarm," from Middle English bugge "specter" (among other things, supposed to scare horses at night); see bug (n.); also compare bogey (n.1), boggart.

The meaning " hesitate, stop as if afraid to proceed in fear of unforeseen difficulties" is from 1630s; the transitive sense of "confound, cause to hesitate" is from 1640s. As a noun from 1650s. Related: Boggled; boggling; boggler (from c. 1600 as "one who hesitates").