‘Pon rep is a multi-usage exclamation, used similarly to “Oh wow!” “No way!” “I swear!” “I promise!” and so forth. The phrase means you swear “upon your reputation,” but the usage varies and is more apt to be used to express surprise or emphasize a point. Using it as a filler is not uncommon, such as “you know.”

The phrase is as likely to be used by gentlemen as by ladies.

If you're a reader of Georgette Heyer, you'll read this Regency cant used quite a lot. One of the best usages can be found in These Old Shades.