Correct spelling, explanation: undecidable is an adjective, but its root is a verb – decide. Both words come from the Latin form decider, which meant to determine. Undecidable probably entered English together with the noun decision, which took place in late Middle English. Today, undecidable is a commonly used adjective.

Definition of undecidable:
1. adjective – when a problem seems to be impossible to solve,
Whether to go to a local college and continue to live with my parents or move to a bigger city seems an undecidable problem to me.

Expressions with undecidable:
A few common expressions with undecidable: undecidable problem, undecidable statement, and undecidable question.
I have no idea whether I should stay or take the new job offer – for me it’s an undecidable problem.
It looks like an undecidable statement and it’s not worth bothering with.
Our teacher gave us some undecidable questions on the test and I spent over 20 minutes on it and came to no conclusion.

Incorrect spelling, explanation: a lot of spelling mistakes in English appear when the -able suffix is added to a word. The spelling of the word usually changes then, but a number of English speakers tend to forget about that. Such a thing happens, for example, in the case of undecidable, which is often misspelled as undecideable. The -e letter at the end of the root word – decide – disappears, when the -able ending is added.