Etymology
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loch (n.)

late 14c., from Gaelic loch "lake, lake-like body," including the narrow, nearly land-locked arms of the sea found in the glacier-scoured landscape of west Scotland; cognate with Old Irish loch "body of water, lake," Breton lagen, Anglo-Irish lough, Latin lacus (see lake (n.1)). "The word was adopted in ONorthumbrian as luh" [OED]. The diminutive form is lochan.

The phrase Loch Ness monster is attested by 1934, the thing itself under slightly different names from 1933. The loch is named for the river Ness that flows out of it at Inverness; the river name is probably from an Old Celtic word meaning "roaring one."

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crack (n.)

"a split, an opening, narrow fracture," mid-15c., earlier "a splitting sound; a fart; the sound of a trumpet" (late 14c.), probably from crack (v.).

Meaning "sharp, resounding blow" is from 1836. Meaning "rock cocaine" is first attested 1985. The superstition that it is bad luck to step on sidewalk cracks has been traced to c. 1890. Meaning "try, attempt" first attested 1830, nautical, probably a hunting metaphor, from slang sense of "fire a gun."

At their head, apart from the rest, was a black bull, who appeared to be their leader; he came roaring along, his tail straight an [sic] end, and at times tossing up the earth with his horns. I never felt such a desire to have a crack at any thing in all my life. He drew nigh the place where I was standing; I raised my beautiful Betsey to my shoulder, took deliberate aim, blazed away, and he roared, and suddenly stopped. ["A Narrative of the Life of David Crockett, Written by Himself," Philadelphia, 1834]

Adjectival meaning "top-notch, superior, excellent, first rate" (as in a crack shot) is slang from 1793, perhaps from earlier verbal sense of "do any thing with quickness or smartness" [Johnson], or from the verb in the sense of "speak boastingly" and suggesting "having qualities to be proud of" [Century Dictionary]. Grose (1796) has "THE CRACK, or ALL THE CRACK. The fashionable theme, the go." To fall or slip through the cracks figuratively, "escape notice," is by 1975. Crack-brained "demented" is attested from 1630s. The biblical crack of doom is in reference to the sound (Old English translates it as swegdynna maest).

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