From Longman Dictionary of Contemporary Englishsomebody could have sworn (that) ...somebody could have sworn (that) ...used to say that someone was sure about something but now they think they were wrong I could have sworn I had my keys. → swear
Examples from the Corpus
somebody could have sworn (that) ...• All of which is very curious we could have sworn Colin Milburn went to good old Greencroft comprehensive.• Athelstan could have sworn he was acting as if there was some one else there.• Corbett could have sworn that momentarily he glimpsed another figure, shadow-like, but fled on.• The friar could have sworn that Sir John was singing a hymn or a song under his breath.• She could have sworn the light had been yellow - pure yellow.• He could have sworn the pile of letters had been deeper, that there had been many more.• No, he recalled other sightings, so real you could have sworn they were alive ... until they vanished.• The Myrcans looked on with what he could have sworn was approval. 