O.E. 1st & 3rd pers. sing. pres. indic. of
cunnan "know, have power to, be able," (also "to have carnal knowledge"), from P.Gmc.
*kunnan "to be mentally able, to have learned" (cf. O.N.
kenna "to know, make known," O.Fris.
kanna "to recognize, admit," Ger.
kennen "to know," Goth.
kannjan "to make known"), from PIE base
*gno- (see
know). Absorbing the third sense of "to know," that of "to know how to do something" (in addition to "to know as a fact" and "to be acquainted with" something or someone"). An O.E. preterite-present verb, its original p.p.,
couth, survives only in its negation (see
uncouth), but cf.
could.
Cannot is attested from c.1400;
can't first recorded 1706 (O.E. expressed the notion by
ne cunnan).