Lewis & Short

mŏnogrammus or -os, -on, adj., = μονόγραμμος, lit. of pictures, that consist of lines merely, outlined, sketched; hence transf.,

  1. I. Of incorporeal gods, shadowy: Epicurus monogrammos Deos et nihil agentes commentus est, Cic. N. D. 2, 23, 59.
    Hence, as subst.: mŏnogrammus, i, m., comically, a skeleton, a shadow: monogrammi dicti sunt homines macie pertenues ac decolores: tractum a picturā, quae priusquam coloribus corporatur, umbra fingitur, Non. 37, 11 sq.; Lucil. ap. Non. l. l.
  2. II. A species of jasper: quae zmaragdo similis traversā lineā albā praecingitur et monogrammos vocatur, Plin. 37, 9, 37, § 118.