Green’s Dictionary of Slang

blab n.

[blab and its v. forms blab and, apparently, blabber are the first sl. terms relating to speech and can be found as such in the 16C. Their history, however, is somewhat older. There is even, according to the OED, a question whether what appears to be an obvious link actually exists. Blab, then spelt blabbe and meaning a ‘chatterer’, occurs in Chaucer c.1374; blab, meaning simply ‘chatter’ or ‘loose talk’, can be found in The Tale of Beryn (c.1400), but then promptly vanishes until the 16C, when it is augmented by a v. form, blab, to chatter (1535). This, in turn, creates a n., blabber, a chatterer. However, the v. blabber predates all these; it occurs in Piers Ploughman (1362) and, with its n. blabberer, is common in the works of John Wyclif (1330–84). Thus, however tempting it may seem, one cannot simply assume that blab is a 14C abbr. of blabber. Instead, the OED suggests, it is related to the noun labbe, a revealer of secrets, in Chaucer, and the verb labbe in Piers Ploughman and to labbyng, open-mouthed. It can also be linked to the Old Dutch labben, to chatter. Thus blab/blabbe might be a mixture of labbe and blabber; but might also be simply onomat.]

1. [mid-16C+] a tell-tale.

2. [mid-19C+] talk.

In derivatives

blabfest (n.) [-fest sfx]

[late 19C+] (US) a gathering where those involved devote themselves to talking, esp. unashamed gossip.

In compounds

blab sheet (n.) [sheet n. (1)]

[1940s–60s] (orig. US black) a newspaper.

In phrases

blabs in labs (n.)

[1970s+] (US campus) a course in linguistics, the ‘labs’ are language laboratories.