in Scandinavian mythology, is a water-wraith or kelpie. There are nicks in sea, lake, river, and waterfall. Both Catholic and Protestant clergy have laboured to stir up an aversion to these beings. They are sometimes represented as half-child, half-horse, the hoofs being reversed, and sometimes as old men sitting on rocks wringing the water from their hair. This kelpie must not be confounded with the nix (q.v.).
Old Nick is the Scandinavian wraith under the form and fashion of an old man. Butler says the word is derived from Nicholas Machiavel, but this can be only a poetical satire, as the term existed many years before the birth of that Florentine.
Nick Machiavel had ne'er a trick (Though he gives name to our old Nick) But was below the least of these.
Old Nick. Grimm says the word Nick is Neken or Nikken, the evil spirit of the North. In Scandinavia there is scarcely a river without its Nikr or wraith. (See Nickar and Nicor. Anglo-Saxon nicor, a monster.)
He nicked it. Won, hit, accomplished it. A nick is a winning throw of dice. Hence Florio (p. 280) says “To tye or nicke a caste of dice.”
To nick the nick. To hit the exact moment. Tallies used to be called “nicksticks.” Hence, to make a record of anything is “to nick it down,” as publicans nick a score on a tally.
In the nick of time. Just at the right moment. The allusion is to tallies marked with nicks or notches. Shakespeare has, “ 'Tis now the prick of noon” (Romeo and Juliet, ii. 4), in allusion to the custom of pricking tallies with a pin, as they do at Cambridge University still. If a man enters chapel just before the doors close, he would be just in time to get nicked or pricked, and would be at the nick or prick of time.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894