défectif

Adjective

 * 1)  defective (said of a verb that does not conjugate itself at all, does not conjugate itself anymore, or that only conjugates itself in certain persons or in certain tenses)

Usage notes
The conjugation can be impossible like falloir or unusual like gésir or even chaloir known today for the expression «peu m’en chaut» (“It is of little import to me”, “I do not care”).