Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
unsound [ʌnˈsaund] adj (health) сла́бый*слаб (floor, foundations) непро́чный*непро́чен (policy) ша́ткий*ша́ток (advice) ненадёжный*ненадёжен
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.