Typically I find hearty to be used as an adjective, for example:
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Thank you for this hearty meal
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He gave a hearty laugh
The definition for which can be found in any dictionary and can mean things like warm-hearted, genuine, forceful, etc.
The noun meaning being
a brave or good fellow, especially with reference to a shipmate.
I couldn't find any reason why it should be especially applied to men of the sea.
Is there a reason why, for pirates in particular, shipmates are hearties?
Best Answer
The OED says
No mention of sea-shanties, and no suggestion that the "hearty work" accompanied by them had any particular relevance. Once again it is a fairly transparent use of a word which to my mind doesn't require any special explanation.
But it clearly is associated with the sea from the beginning: the earliest citation is from Phantom Ship by F Marryat in 1839.