having nothing to be ashamed of before heaven and earth
A yojijukugo expressing a clear conscience, as if one can look up to heaven and down to earth without shame. Very rare and literary; not used in everyday conversation.
The phrase 俯仰天地に愧じず expresses having nothing to be ashamed of in one's heart.
From Chinese classical literature, combining 俯 (look down), 仰 (look up), and 天地 (heaven and earth). The exact historical derivation is uncertain, but it is a fixed yojijukugo expressing moral integrity.