The Collaborative International Dictionary of English v.0.48:

Packwax \Pack"wax`\ (p[a^]k"w[a^]ks`), n. (Anat.)
   Same as Paxwax.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Paxwax \Pax"wax`\, n. [For faxvax, fr. AS. feax hair (akin to
   OHG. fahs) + weaxan to grow. See Wax to grow, and cf.
   Faxed, Pectinate.] (Anat.)
   The strong ligament of the back of the neck in quadrupeds. It
   connects the back of the skull with dorsal spines of the
   cervical vertebr[ae], and helps to support the head. Called
   also paxywaxy and packwax.
   [1913 Webster]