Jancey is extremely rare in the UK as a first name, so you're one of a select few. My guess would be that it's from the surname - a long-established one which seems to occur with that spelling mainly in the Midlands and around the Welsh borders. To my knowledge there are no French names or places which sound or look anything like it, except - big stretch here - the town & lake of Annecy (pronounced an see) in eastern France, but I can't see that as a likely origin.