Webster's 1828 Dictionary
CANNOT
[can and not. ] These words are usually united, but perhaps without good reason; canst and not are never united.
Webster's 1913 Dictionary
CANNOT
Can "not. Etym: [Can to be able _ -not. ]
Defn: Am, is, or are, not able; -- written either as one word or two.
New American Oxford Dictionary
cannot
can not |kəˈnät, ˈkanˌät ˈkæˌnɑt | ▶contraction can not. usage: Both the one-word form cannot and the two-word form can not are acceptable, but cannot is more common (in the Oxford English Corpus, three times as common ). The two-word form is better only in a construction in which not is part of a set phrase, such as ‘not only … but (also )’: Paul can not only sing well, but he also paints brilliantly.
Oxford Dictionary
cannot
can ¦not |ˈkanɒt, kaˈnɒt | ▶contraction can not. usage: Both the one-word form cannot and the two-word form can not are acceptable, but cannot is more common (in the Oxford English Corpus, three times as common ). The two-word form is better only in a construction in which not is part of a set phrase, such as ‘not only … but (also )’: Paul can not only sing well, he also paints brilliantly.
Sanseido Wisdom Dictionary
cannot
can not /kǽnɑt |-nɔt / (!⦅主にかたく ⦆) canの否定形 (!can notともつづるがcannotの方が一般的; can'tは主に ⦅話 /くだけた書 ⦆; →can 1 語法 ) .