mid-14c., plegge, "surety, bail," from Old French plege (Modern French pleige) "hostage, security, bail," also Anglo-Latin plegium, both probably from Frankish *plegan "to guarantee," from *pleg-, a West Germanic root meaning "have responsibility for" (source also of Old Saxon plegan "vouch for," Middle Dutch plien "to answer for, guarantee," Old High German pflegan "to care for, be accustomed to," Old English pleon "to risk the loss of, expose to danger"), from PIE root *dlegh- "to engage oneself, be or become fixed" [Watkins].
From late 14c. as "person who goes surety or gives bail for another;" late 15c. (Caxton) as "personal property given as surety for a debt or engagement. By 1520s as "a token or sign of favor, agreement, etc.
Meaning "allegiance vow attested by drinking with another" is from 1630s. Sense of "solemn promise, one's word given or considered as security for the performance (or refraining from) an act" is recorded by 1814, though this notion is from 16c. in the verb. Weekley notes the "curious contradiction" in pledge (v.) "to toast with a drink" (1540s) and pledge (n.) "the vow to abstain from drinking" (1833). Meaning "student who has agreed to join a fraternity or sorority" dates from 1901.