The word gratitude traces its roots to the Latin word gratus, which means "pleasing" or "thankful".  It entered the English language in the mid-15th century via Old French, derived from the Medieval Latin gratitudo ("thankfulness").  The ending -tūdo (or -itude) is a Latin suffix used to turn adjectives into abstract nouns indicating a state or condition.  

According to the Oxford English Dictionary (OED), the English word "gratitude" was first recorded in the early 1500s.  The earliest known use of the noun appears before 1513 in the writings of the Scottish poet and courtier William Dunbar.