The Collaborative International Dictionary
Not but
Not \Not\, adv. [OE. not, noht, nought, naught, the same word as E. naught. See Naught.] A word used to express negation, prohibition, denial, or refusal.
Not one word spake he more than was need.
--Chaucer.
Thou shalt not steal.
--Ex. xx. 15.
Thine eyes are upon me, and I am not.
--Job vii. 8.
The question is, may I do it, or may I not do it?
--Bp.
Sanderson.
Not . . . but, or Not but, only. [Obs. or Colloq.]
--Chaucer.