Definify.com

Webster 1913 Edition

Adj.

Not advisable; inadvisable; inexpedient.

Lowth.

–

Unˊad-vis′a-bly

adv.

Adj.

s as z. Not advisable; not to be recommended; not expedient; not prudent.

unadvisable ‎(comparative more unadvisable, superlative most unadvisable)

Inadvisable.
- 1948, William Burroughs, letter, 5 Jun 1948:
  Will go back there to get it in a week or so, but may find the harvest unadvisable for reasons of security.
(archaic) Unwilling to take advice.
(archaic) Imprudent.

"Inadvisable" is used about 10 times more frequently than "unadvisable" at COCA (US usage) and 20 times more at BNC (UK written usage).