"Dingy," "Dinghy," and a Ring-a-Ding-Ding!

     
       I see the word "dingy" and wonder how we know how to pronounce it, especially when the word "dinghy" is pronounced [DING-ee].  Why shouldn't "dingy" be pronounced [DING-ee], or, both words [DIN-jee]?

       "Dinghy" is from Hindi and means a little boat.  The OED says it's pronounced [DING-ghee] in Britain, spelled with "gh" in English (no "h" in the Indian word) to indicate the "g" is hard.  I ask why?  "gh" could be [f] as in "rough," in which case [DIN-fee]; or silent as in "doughy," in which case [DIN-ee]. 

       "Dingy" means dirty, and the origin is either "unknown" (my preferred one) or related to "dung."  But how do I know this is pronounced [DIN-jee] and not [DING-ee] as with "thingy" or "slangy" or, one might repeat, "dinghy"?