In Gildersleeves Latin Grammar you can find declension charts with the word
wanting inserted in 3 places
I II III Nom. a. us (os) ; wanting ; um (om). s ; wanting. Gen. ae (ās, āī, āi). ī (ēī). is (us, es). Dat. ae (āī). ō. ī (ēī, i). Acc. am. um (om). em, im. Voc. a. e ; wanting ; um (om). s. Abl. ā (ād). ō (ōd). e, ī (ēd, īd).
(The plurals are ommitted since it does not appear there.)
I'm wondering what this
wanting is supposed to indicate. I ran a find for the word wanting through the entire text. There is no explanation as to what this means anywhere as far as I can find. It does seem to be used elsewhere to mark forms that are missing, but that doesn't really make sense here.
The only hypothesis I have currently is that it is that for the 2nd declension it marks the locations at which non-o-stem nouns such as puer, remain uninflected. And that lines up, but if that were true I would also expect a
wanting to be present in the Vocative singular for third declension as well, since 3rd declension nouns have the same nominative and vocative forms. But it is not.
What is this there for?
The entire the 3rd edition of this text can be viewed on archive.org the chart in question is on page 13.