As a noun, freak is documented from the 1560s with the meaning “sudden turn of mind”:
The king, in a freak of anger, ordered the general’s execution.
From meaning “a sudden turn of mind,” freak came to mean “a trick” or “a prank”:
The boy was expelled for some boyish freak.
By the 18th century, freak could mean something extremely imaginative. The 1785 citation in the OED refers to a wonderful ice-palace as a “freak.”
The expression freak of nature gained currency in the 19th century. From that phrase derives freak in the sense of “something that has developed abnormally,” like a two-headed calf.
In modern informal speech, freak is used with a qualifying word to label someone as being “extremely committed to something”:
The earliest example of this formation is from 1908: camera buffs were called kodak freaks.
Freak is also used as a verb:
to freak out: (occas. without out): to undergo an intense emotional experience, to become stimulated, to rave, esp. under the influence of hallucinatory drugs. Also trans., to cause (a person) to be aroused or stimulated in such a way. (OED)
The verb has in turn spawned the noun freak-out (also spelled without the hyphen):
Horse had a freak out in the trailer and cut hocks.
Freak has two adjective forms, freakish (1653) and freaky (1824). The highway official being quoted in a news story about the narrow escape of a motorist when a second lane marker came loose from the road must have been too rattled to think of either of these adjectives:
… this is just another freak accident, even freaker than before