Wikipedia (
https://en.wikipedia.org/wiki/FERT):
"The meaning of the letters has been a matter of some controversy, to which a number of interpretations have been offered. The motto is believed an acronym of:[4]
Foedere et Religione Tenemur (Latin: "We are bound by treaty and by religion");
Fortitudo Eius Rhodum Tenuit (Latin: "His strength conquered Rhodes" or "By his bravery he held [or occupied] Rhodes"), referring to the victory of Amadeus V, Count of Savoy (1249–1323), who fought against the Saracens at the 1315 siege of Rhodes;[2] or either
Fortitudo Eius Rempublicam Tenet (Latin: "His bravery [or strength] preserves [or defends] the state"); or
Fides Est Regni Tutela (Latin: "Faith is the protector of [our] Kingdom").
It has also been suggested that the letters are actually the Latin word fert (third-person singular present active indicative of ferre), meaning "[he/she/it] suffers/bears", possibly referring to Jesus bearing the sins of the world."