The only free and comprehensive online etymological dictionary of the Spanish language
libertad f. (Noun) "liberty"
13th cent. From libertas 'id.,' from liber "free" and -tas (see libre and -tad respectively).
Portuguese liberdade, Galician liberdade, Catalan llibretat, French liberté, Italian libertà, Romanian libertate

"Although the opposition “free/slave” is common to all Indo-European peoples, a common designation of the notion of “liberty” is unknown." ~ E. Benveniste, Indo-European Language and Society (1973)