'to perk up one's ears' definitions:

Definition of 'to perk up one's ears'

From: GCIDE

Perk \Perk\ (p[~e]rk), v. t. [imp. & p. p. Perked (p[~e]rkt); p. pr. & vb. n. Perking.] [Cf. W. percu to trim, to make smart.] To make trim or smart; to straighten up; to erect; to make a jaunty or saucy display of; as, to perk the ears; to perk up one's head. --Cowper. --Sherburne. [1913 Webster]
to perk up one's ears to begin to listen attentively; -- usually used when something of interest is mentioned in some discourse, after one's attention had lapsed; as, he perked up his ears when he heard his name mentioned. [PJC]