blab (v.)

mid-15c., blabben, "to talk idly and foolishly, talk too much," apparently from Middle English noun blabbe "one who does not control his tongue" (late 13c.), which probably is echoic (compare Old Norse blabbra, Danish blabbre "babble," German plappern "to babble"). It is attested from c. 1600 as "to talk indiscreetly." Related: Blabbed; blabbing.

The exact relationship between the noun and verb blab and blabber is difficult to determine. The noun was "[e]xceedingly common in 16th and 17th c.; unusual in literature since c 1750" [OED]. Middle English also had lab (v.) "talk foolishly, let out a secret" (late 14c.), which is said to be from Low German; hence also labster (Middle English labestere, late 13c.) "female gossip, a scold."