a US financial institution which buys and sells loans and mortgages to banks and other institutions, but does not deal directly with ordinary borrowers. Its full name is the Federal National Mortgage Association (FNMA).
From Longman Business Dictionary
Fannie MaeFan·nie Mae /ˌfæni ˈmeɪ/ noun
1ORGANIZATIONSFINANCEBANKINGan informal name for the FEDERAL NATIONAL MORTGAGE ASSOCIATION
2[countableC usually plural]FINANCE bonds issued by the Fannie Mae