From etymonline.com:
Also seems like there should be some connection between pledge and fledgeling...1348, "surety, bail," from O.Fr. plege (Fr. pleige) "hostage, security, bail," probably from Frank. *plegan "to guarantee," from a W.Gmc. root meaning "have responsibility for" (cf. O.Saxon plegan "vouch for," O.H.G. pflegan "to care for, be accustomed to," O.E. pleon "to risk, expose to danger"). Meaning "allegiance vow attested by drinking with another" is from 1635. Sense of "solemn promise" first recorded 1814, though this meaning is from c.1400 in the verb. Weekley notes the "curious contradiction" in pledge "to toast with a drink" (1546) and pledge "the vow to abstain from drinking" (1833). Noun meaning "student who has agreed to join a fraternity or sorority" dates from 1901.
-Tim