This interesting surname, of medieval Scottish origin, though thought by many to be a nickname for a bow-legged person, from the older Scots "cruik", hook or bend, plus "shank", leg-bone, is, in fact, of locational origin from residence by the river Crui(c)k in the former county of Kincardine (now part of the Grampian region). The river Crui(c)k, which rises in the parish of Fearn and joins the North Esk near the Kirk of Stracathro, is so called from the element, "cruik" (as above), probably with reference to the winding course taken by the river. The "shank" comes from the topographical reference to a "projecting point of a hill joining it to the plain".

Here is an explanation of the origins from,

The Surnames of Scotland, Their Origin, Meaning and History by George F. Black, Ph.D., 1996

"With the possible exception of the first record of this name, which may point to a nickname, I do not think this surname has any connection with bowleggedness or 'crooked shanks.' The earliest spellings of the name, with the one exception noted, are always in the singular. The two counties with which the name is most intimately connected are Kincardine and Aberdeen, and in the former we have the river Cruick rising in the parish of Fearn and joining the North Esk near the Kirk of Stracathro. The word 'Cruick' is found several times in combination in Scots place names, i.e. 'Cruikitheuch' .. 'the crukit aiker'  .. Cruiktheugh, later  ... Cruikhaugh at Inverbervie. "Shank' is also a common element in Scots place names, either alone or in combination, i.e. Schank (in Midlothian, whence the surname Shank or Schank); Shankend, Shankfoot, etc. In toponomy 'shank' has the meaning of 'projecting point of a hill joining it to the plain.' The surname may thus quite well be of loacl origin - the shank on the Cruick or the Cruick-shank." An early record is John Crokeshanks, burgess of Haddington, rendered homage, 1296...."

and another from,

Craigellachie, A Quarterly Publication of Clan Grant Society USA, Inc., Volume XXXI, Issue 2, Summer 2008
From the Article: Some Septs of the Clan Grant – Part II - page 8, by James Grant, Historian

"Cruickshank, Crookshank (frequently pronounced KROO shak) is a name of undetermined origin.
Its original connotation was assumed to mean “crooked leg,” but more recently, the name has been
linked to a geographical location in Aberdeenshire. John Cruickshank was a tenant in Achnahandet, Duthil, in 1671. The same man or another of the same name was in Cromdale in 1708. James Cruishank was the “malt man” at Castle Grant in the 18th century. A number of Cruickshanks are noted in the parish records of Duthil and Cromdale in the 18th century. Andrew Crukshank was a weaver in Gortons circa 1730. Andrew Grant or Cruikshank was baptized in Cromdale in 1733. Crookshanks and Cruickshanks were in Glenlivet and the parish of Kirkmichael in the 18th century. Hector Cruikshank in Delavorar, Kirkmichael, was a Jacobite in the ’45. Alexander Cruickshank, Sergeant in the Black Watch was buried
at Inverallan in 1863".

Other early references include:

Christin Crukschank was mentioned in a foundation charter of the chapel of Urchany in 1334, and Cristinus Cru(k)sank was admitted burgess of Aberdeen in 1408. Adam Crukshank was vicar of Crovdane in 1414, and John Crewkshanks was a tailor in Aberdeen in the year 1688. In modern times, the surname is also spelt Cruickshanks, Cruikshank, and Crookshanks.

On September 27th 1750 Alexander Cruickshank and Margaret Kemlo were married in Glenbervie, Kincardine. A Coat of Arms granted to the Cruickshank family of Tillymorgan, Aberdeenshire, is gold, three black boar's heads couped.

The first recorded spelling of the family name is shown to be that of John Crokeshanks, burgess of Haddington, which was dated 1296 - "Calendar of Documents relating to Scotland", during the reign of King John Balliol, King of Scotland, 1292-1296.

Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.