From a misinterpretation of a-missing (a- + missing) as a single word.
amissing (comparative more amissing, superlative most amissing)
- Missing, lacking.
1888, Rudyard Kipling, ‘Dray Wara Yow Dee’, Black and White, volume 1, Folio Society, published 2004, page 369:
Kamal is dead now, but his nephew has taken up the matter, and there will be more horses amissing if the Khaiber Levies do not look to it.