before 900; Middle English, Old English, present indicative singular 1st, 3rd person of cunnan to know, know how; cognate with German, Old Norse, Gothic kann; see ken, know¹

Can1 and may1 are frequently but not always interchangeable in senses indicating possibility: A power failure can (or may ) occur at any time. Despite the insistence by some, that can means only “to be able” and may means “to be permitted,” both are regularly used in seeking or granting permission: Can (or May ) I borrow your tape recorder? You can (or may ) use it tomorrow. Sentences using can occur chiefly in spoken English. May in this sense occurs more frequently in formal contexts: May I address the court, Your Honor? In negative constructions, can't or cannot is more common than may not : You can't have it today. I need it myself. The contraction mayn't is rare.
Can but and cannot but are formal and now somewhat old-fashioned expressions suggesting that there is no possible alternative to doing something. Can but is equivalent to can only : We can but do our best. Cannot but is the equivalent of cannot help but : We cannot but protest against these injustices. See also cannot, help.

before 1000; Middle English, Old English canne, cognate with German Kanne, Old Norse kanna, all perhaps < West Germanic; compare Late Latin canna small vessel