c.1400, "to promise" (something to someone), "to give over as security for repayment," also "promise faith to," from pledge, n. and from Old French plegier, from plege (n.). From mid-15c. as "to stand surety for, be responsible for;" late 15c. as "to mortgage." Meaning "put (someone) under oath" is from 1570s; sense of "to solemnly promise or guarantee" is from 1590s, as is sense "to drink a toast." Related: Pledged; pledging.

mid-14c., "surety, bail," from Old French plege (Modern French pleige) "hostage, security, bail," probably from Frankish *plegan "to guarantee," from *pleg-, a West Germanic root meaning "have responsibility for" (cognates: 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;" see plight, v.).Meaning "allegiance vow attested by drinking with another" is from 1630s. Sense of "solemn promise" first recorded 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.