The Collaborative International Dictionary of English v.0.48:

Rancescent \Ran*ces"cent\ (r[a^]n*s[e^]s"sent), a. [L.
   rancescens, p. pr. of rancescere, v. incho. from rancere to
   be rancid.]
   Becoming rancid or sour.
   [1913 Webster]