Rembourser is the French infinitive for to reimburse or to pay back, and online the clip is used less for language learning than for comic timing. Short spoken French words often circulate as reaction sounds because their meaning is clear from context even to non French speakers when paired with money imagery. This one has settled into refund and payment humour, appearing in edits about bad purchases, split bills and failed transactions, where a single formal sounding word does the work of a whole complaint about cost.