And thanks, and ever thanks; and oft good turns
Are shuffled off with such uncurrent pay

Antonio has volunteered to be Sebastian's servant; Sebastian has accepted the offer, but he is embarrassed that he can't do anything for Antonio except say "thanks." A gentleman, which Sebastian is, ought to pay his servants with money, not just the word "thanks." The word "thanks" is "uncurrent pay" because nothing can be bought with it. We still use the word "currency" to designate money which is still in circulation, as opposed to old coins which have only sentimental or historical value.