insoluble
inextricable
unsolvable
astrophysical
chicken-and-egg
excus
grief-worn
irresolvable
locked-room
misleading
never-forgotten
principalagent
propound
sly-looking
two-horned
unapprehensive
undeterminable
undogmatic
unfits
unguessable
unoffended
westernized
wittier
soluble
solvable