hardhead vs steelhead

hardhead

noun
  • A brown diving duck, Aythya australis, native to Australia. 

  • Any of certain saltwater sciaenid (Sciaenidae) fishes. 

  • Any of several weeds in or formerly classified in the genus Centaurea, particularly Russian knapweed (Acroptilon repens). 

  • A Scottish billon coin circulated in the reigns of Mary and James VI. 

  • Residual ore that failed to melt during a smelting process. 

  • One who is practical or hardheaded. 

  • Mylopharodon conocephalus of fresh waters in California. 

steelhead

noun
  • The ruddy duck. 

  • The anadromous form of the rainbow trout, Oncorhynchus mykiss. 

How often have the words hardhead and steelhead occurred in a corpus of books? (source: Google Ngram Viewer )