mid-13c., from Old English hlid "lid, cover, opening, gate," from Proto-Germanic *khlithan (cf. Old Norse hlið "gate, gap," Swedish lid "gate," Old French hlid, Middle Dutch lit, Dutch lid, Old High German hlit "lid, cover"), from PIE root *klei- "to lean" (see lean (v.)), with here perhaps the sense of "that which bends over." Meaning "eyelid" is from early 13c. Slang sense of "hat, cap" is attested from 1896. Slang phrase put a lid on "clamp down on, silence, end" is from 1906.
To expose a scandal, esp political or governmental corruption (1920s+)