pennig
This is Early Noldorin panthig (PE13:124) imported into later Sindarin with the appropriate sound changes. It is formed from pant 'money' with the singulative suffix -ig.
Note the similarity to English penny (Old English peni(n)g), Dutch penning, German Pfennig and so on.
EN panthig 'coin' (PE13:124)
S. mirian and canath (PM:45) are specific coins from Gondor.
Roman Rausch