Rooted in 《
诗经·
小雅·
正月》 (Book of Odes): '
谓天盖高，
不敢不局；
谓地盖厚，
不敢不脊' — though the heavens be high, one dares not stand straight; though the earth be thick, one dares not tread heavily. Most often heard in the negative form
不知天高地厚 ('doesn't know how high the sky is or how thick the earth is') — i.e., reckless, full of themselves.
