Welsh and French short form of Jacques/Jacob, from Hebrew Ya'akov meaning 'supplanter'.
Jac is the Welsh form of Jack, and through Jack the name connects to the great river of Jacob — the Hebrew Ya'akov, meaning "supplanter" or, in a more charitable reading, "he who follows at the heel." Jacob is one of the founding patriarchs of the Hebrew Bible, and his story of wrestling with an angel until dawn — and refusing to release his grip until he received a blessing — has made the name a perennial symbol of tenacious, sometimes morally ambiguous striving. From Jacob came the Latin Jacobus, the medieval French Jacques, the English James, and ultimately the familiar Jack.
In Welsh, Jac is simply the native orthography, and it has been used in Wales for centuries without any sense of abbreviation — it stands as a full given name in its own right, not a diminutive. Wales has a strong tradition of anglicized names being reclaimed in their Cymraeg forms, and Jac sits alongside Tomos, Dafydd, and Rhys as an example of this quiet linguistic assertion. The name appears in Welsh folk songs and throughout Welsh-language literature as an everyman figure, warm and dependable.
Outside Wales, Jac reads to English speakers as a purposeful respelling — minimalist, artisanal, with a slight Continental feel reminiscent of French Jacques. It has attracted interest from parents who find plain Jack too ubiquitous but want to retain its solid, unshowy character. The single-letter substitution accomplishes something typographically interesting: it makes a familiar name look newly crafted without making it unpronounceable, which is a difficult balance to strike.