The prefix of the word "inadmissible" is "in-," which means "not" or "without." In this case, "inadmissible" means not able to be accepted or allowed.

Some evidence is inadmissible in a court of law.

Inadmissible Evidence - film - was created in 1968.

Something that is unacceptable or can not be permitted to happen. If my advice is inadmissible, you couldn't use it.

Inadmissible expenses are expense not allowed for deducting from some figure for arriving on some acceptable figure e.g for taxation purposes depreciation is an inadmissible expense.