I thought it might be a vulgarization for an inflection of shoeing ("to put horseshoes") , but it is for errar ( "equivocar" ) , for a common resource when pronouncing two equal vowels at the end of one word and the beginning of the next which is to insert an aspirated /j/ and thus mark more clearly the separation between the two. The verb 'jerrar' did not exist, but it was created for these specific cases and surely today it has become popular. See and err .
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