The Collaborative International Dictionary of English v.0.48:

Astert \A*stert\, v. t. [Pref. a- + start; OE. asterten,
   asturten.]
   To start up; to befall; to escape; to shun. [Obs.] --Spenser.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Astert \A*stert"\, v. i.
   To escape. [Obs.] --Chaucer.
   [1913 Webster] Asthenia