Multiple sides of bacon or pork; multiple slabs or pieces of timber.
From 'flitch' plus regular plural '-es' (Old English '-as'), the standard way to pluralize most nouns in English.
English's '-es' plural is a simplified descendant of Old English's complex noun system—we've traded multiple different plural endings for just one '-s' or '-es,' making the language's grammar much simpler than it was a thousand years ago.
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