off-peak (adj.)

by c. 1200 as an emphatic form of Old English of (see of), employed in the adverbial use of that word. The prepositional meaning "away from" and the adjectival sense of "farther" were not firmly fixed in this variant until 17c., but once they were they left the original of with the transferred and weakened senses of the word. Meaning "not working" is from 1861.

Off the cuff "extemporaneously, without preparation" (1938) is from the notion of speaking from notes written in haste on one's shirt cuffs. In reference to clothing, off the rack (adj.) "not tailored, not made to individual requirements, ready-made" is by 1963, on the notion of buying it from the rack of a clothing store; off the record "not to be publicly disclosed" is from 1933; off the wall "crazy" is 1968, probably from the notion of a lunatic "bouncing off the walls" or else in reference to carom shots in squash, handball, etc.

1520s, "pointed top, projecting summit," a variant of pike (n.4) "sharp point." Meaning "top of a mountain, a precipitous mountain with a more or less conical summit" is recorded by 1630s, though pike was used in this sense c. 1400. Figurative sense is 1784. Of beards, 1590s; of hats, 1650s. Meaning "point formed by hair on the forehead" is from 1833. As "the highest point" in any varying quantity, or the time when this occurs, by 1902. 

The Peak, the prominent hill in Derbyshire, England, is older than the word for "mountaintop;" compare Old English Peaclond, for the district, Pecsaetan, for the people who settled there, Peaces ærs for Peak Cavern. In this case it is sometimes said to be a reference to an elf-denizen Peac "Puck."