The word 'vac' is a 20th-century British English slang abbreviation of 'vacation'. 'Vacation' itself entered English in the late 14th century from Old French vacacion, meaning "vacancy, vacant position", which derived from Latin vacationem (nominative vacatio), meaning "leisure, freedom, exemption, being free from duty". This Latin noun comes from the past participle stem of the verb vacare, "to be empty, free, or at leisure". The core concept, traceable to the Proto-Indo-European root eue-, meaning "to leave, abandon, go out", evolved from the literal state of being empty or free to the modern abstract meaning of a period of freedom from work or study.

This word is a clipping, a shortened form of the longer word "vacation". It is not composed of productive morphemes in its current form.