From (the alternate of) opheilo; something owed, i.e. (figuratively) a due; morally, a fault -- debt.
see GREEK opheilo
a. properly, that which is justly or legally due, a debt; so for מַשָּׁאָה, Deuteronomy 24:12 (10); ἀφιέναι, 1 Macc. 15:8; ἀποτίνειν, Plato, legg. 4, p. 717 b.; ἀποδιδόναι, Aristotle, eth. Nic. 9, 2, 5 (p. 1165a, 3). κατά ὀφείλημα, as of debt, Romans 4:4.
b. in imitation of the Chaldean חוב or חובָא (which denotes both debt and sin), metaphorically, offence, sin (see ὀφειλέτης, b.); hence, ἀφιέναι τίνι τά ὀφειλετα αὐτοῦ, to remit the penalty of one's sins, to forgive them, (Chaldean חובִין שְׁבַק), Matthew 6:12. (Cf. Winer's Grammar, 30, 32, 33.)