a way of writing can not, c. 1400, from can (v.1) + not. Old English expressed the notion by ne cunnan. The typical representation of the Scottish pronunciation is canna.
Entries linking to cannot
can v.1
Old English 1st and 3rd person singular present indicative of cunnan "to know," less commonly as an auxiliary, "have power to, to be able," (also "have carnal knowledge"), from Proto-Germanic *kunnjanan "be mentally able, have learned" (source also of Old Norse kenna "become acquainted, try," Old Frisian kanna "recognize, admit, know," German kennen "know," Middle Dutch kennen "know," Gothic kannjan "make known"), from PIE root *gno- "to know."
It holds now only the third sense of "know," that of "know how to do something" (as opposed to "know as a fact" and "be acquainted with" something or someone). Also used in the sense of may, denoting mere permission. An Old English preterite-present verb, its original past participle, couth, survived only in negation (see uncouth), but compare could. The present participle has spun off with a deflected sense as cunning.
not adv.
negative particle, a word expressing negation, denial, refusal, or prohibition, mid-13c., unstressed variant of noht, naht "in no way" (see naught). As an interjection to negate what was said before or reveal it as sarcasm, it is attested by 1900, popularized 1989 by "Wayne's World" sketches on "Saturday Night Live" TV show.
Not, spoken with emphasis, often stands for the negation of a whole sentence referred to: as, I hope not (that is, I hope that the state of things you describe does not exist). [Century Dictionary, 1895]
To not know X from Y (one's ass from one's elbow, shit from Shinola, etc.) was a construction attested from c. 1930 in modern use; but compare Middle English not know an A from a windmill (c. 1400). Double negative construction not un- was derided by Orwell, but is persistent and ancient in English, popular with Milton and the Anglo-Saxon poets.